Hydraulic Strut Assembly for Semi-Levered Landing Gear

ABSTRACT

A hydraulic strut assembly, for use in a semi-levered landing gear in an aircraft, comprising an actuator and a manifold associated with the actuator. The actuator comprises a housing, a first piston, a second piston, and a third piston. The first piston is positioned between outer and inner cylindrical structures of the housing. The outer and inner cylindrical structures and first piston form an outer chamber that receives a first fluid. The inner cylindrical structure, the first piston, and the second piston, which is nested within the first piston, form an inner chamber, which holds a second fluid comprising a gas. A volume of the inner chamber changes when at least one of the first and second pistons moves. The third piston is positioned between the outer cylindrical structure and the first piston. The first, second, and third pistons move in a direction parallel to an axis through the housing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/951,861, filed Nov. 22, 2010, entitled “Hydraulic Actuatorfor Semi Levered Landing Gear”, which is incorporated herein byreference.

BACKGROUND INFORMATION

1. Field

Embodiments of the present disclosure relate generally to landing gearand, more particularly, to a semi-levered landing gear and an associatedmethod of positioning the bogie beam of the landing gear using atelescopic hydraulic actuator.

2. Background

Many airplanes include landing gear to facilitate takeoff, landing andtaxi. The landing gear of some aircraft includes a shock absorber thatis pivotally attached to a bogie beam at a distal or lower end thereof.The shock absorber may also be referred to as a shock strut. The bogiebeam includes two or more axles upon which tires are mounted. In thisregard, the bogie beam may include a forward axle positioned forward ofthe shock absorber and an aft axle positioned aft of the shock absorber.Upon takeoff, an airplane having a conventional landing gear withforward and aft axles will pivot about the pin that attaches the bogiebeam to the shock absorber such that all of the landing gear tires havean equal load distribution.

In order to provide additional ground clearance for rotation of theaircraft during takeoff, semi-levered landing gear mechanisms have beendeveloped. A semi-levered landing gear fixedly positions the shockabsorber and the forward end of the bogie beam during takeoff such thatthe forward axle is in a raised position relative to the aft axle whenthe airplane has left the ground. As such, the aircraft pivots about theaft axle, rather than the pin that pivotally connects the bogie beam tothe shock absorber, provided that the extend pressure of the shockabsorber has been increased sufficiently. By rotating about the aftaxle, the landing gear height is effectively increased so as to provideadditional ground clearance for rotation of the aircraft during takeoff.As a result, the takeoff field length (TOFL) of the aircraft may bereduced, the thrust used by the engines may be reduced, or the weightcarried by the aircraft may be increased while maintaining the sametakeoff field length.

In order to provide for rotation of the aircraft about the aft axleduring takeoff, a semi-levered landing gear locks the bogie beam in a“toes-up” attitude such that the tires mounted upon the aft axle supportthe aircraft, while the tires mounted upon the forward axle are raisedabove the surface of the runway. Following takeoff, the landing gear isgenerally stowed in a location such as a wheel well. In order to fitwithin a conventional wheel well, the landing gear is typically unlockedand the bogie beam repositioned in a “stowed” attitude prior toretracting the landing gear into the wheel well. Thereafter, duringlanding, the landing gear is lowered and the bogie beam is repositionedsuch that the forward axle is higher than the aft axle. Upon touch down,all of the wheels, including both those on the forward axle and the aftaxle, equally bear the weight of the aircraft. Typically, the lockingand unlocking of a semi-levered gear system, and the resultingrepositioning of the bogie beam relative to the shock absorber, occurswithout input from the pilot or the flight control system.

One type of semi-levered landing gear utilizes a mechanical linkage tolock the bogie beam during takeoff, but uses a separate mechanicallinkage, termed a shrink-link, to reposition the shock absorber forretraction into the wheel well. The use of a shrink-link increases thecomplexity, expense and weight of the resulting semi-levered landinggear more than desired. Mechanical linkages also may not providesufficiently desired damping during landing or bogie beam pitchdampening while on the ground.

Another type of semi-levered landing gear includes a locking hydraulicstrut to lock the bogie beam in the desired orientation for takeoff. Thelocking hydraulic strut is essentially a locking actuator, but has anumber of additional chambers and an internal floating piston. While asemi-levered landing gear having a locking hydraulic strut is suitablefor some aircraft, the landing gear of other aircraft may not havesufficient clearance or room for the hydraulic strut to be positionedbetween the shock absorber and the bogie beam in an efficient manner.

Accordingly, it would be desirable to provide an improved semi-leveredlanding gear hydraulic actuator that may be used on landing gears thatdo not have sufficient space for housing a conventional lockinghydraulic strut configuration. In particular, it would be desirable toprovide a semi-levered landing gear that is both weight and costefficient and that is not overly complex, while still satisfying thevarious operational requirements of the semi-levered landing gear.

SUMMARY

In one illustrative embodiment, a hydraulic strut assembly comprises ahousing, a first piston, a second piston, and a third piston. Thehousing comprises an outer cylindrical structure and an innercylindrical structure. The first piston is positioned between the outercylindrical structure and the inner cylindrical structure. An outerchamber is configured to receive a first fluid is formed between theouter cylindrical structure, the inner cylindrical structure, and thefirst piston. The second piston is nested within the first piston. Theinner cylindrical structure, the first piston, and the second pistonform an inner chamber in which a volume of the inner chamber changeswhen at least one of the first piston and the second piston move. Theinner chamber is configured to hold a second fluid comprising a gas. Thethird piston is positioned between the outer cylindrical structure andthe first piston. The first piston, the second piston and the thirdpiston are configured to move in a direction parallel to an axis throughthe housing.

In another illustrative embodiment, an actuator for use in a hydraulicstrut assembly comprises a housing, a first piston, a second piston, anda third piston. The housing comprises an outer cylindrical structure andan inner cylindrical structure. The first piston is positioned betweenthe outer cylindrical structure and the inner cylindrical structure. Theouter chamber is configured to receive a first fluid that is formedbetween the outer cylindrical structure, the inner cylindricalstructure, and the first piston in which the first fluid comprises ahydraulic liquid. The second piston is nested within the first piston.The inner cylindrical structure, the first piston, and the second pistonform an inner chamber in which a volume of the inner chamber changeswhen at least one of the first piston and the second piston move. Theinner chamber is configured to hold a second fluid comprising thehydraulic liquid and a gas. The third piston is positioned between theouter cylindrical structure and the first piston. The first piston, thesecond piston and the third piston are configured to move in a directionparallel to an axis through the housing.

In yet another illustrative embodiment, a method for operating anaircraft to perform an alternate landing is present. The aircraft isoperated to perform the alternate landing. An actuator in a landing gearassembly for the aircraft comprises a housing, a first piston, a secondpiston, and a third piston. The housing comprises an outer cylindricalstructure and an inner cylindrical structure. The first piston ispositioned between the outer cylindrical structure and the innercylindrical structure. An outer chamber is configured to receive a firstfluid that is formed between the outer cylindrical structure, the innercylindrical structure, and the first piston. The second piston is nestedwithin the first piston. The inner cylindrical structure, the firstpiston, and the second piston form an inner chamber in which a volume ofthe inner chamber changes when at least one of the first piston and thesecond piston move. The inner chamber is configured to hold a secondfluid comprising a gas. The third piston is positioned between the outercylindrical structure and the first piston. The first piston, the secondpiston and the third piston are configured to move in a directionparallel to an axis through the housing. The second piston and the firstpiston are retracted in response to a load being applied to the secondpiston when the landing gear assembly contacts a ground on which theaircraft is landing. The gas in the inner chamber compresses when thesecond piston retracts.

The features and functions can be achieved independently in variousillustrative embodiments of the present disclosure or may be combined inyet other illustrative embodiments in which further details can be seenwith reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the illustrativeembodiments are set forth in the appended claims. The illustrativeembodiments, however, as well as a preferred mode of use, furtherobjectives and features thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment of thepresent disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is an illustration of an aircraft in the form of a block diagramin accordance with an illustrative embodiment;

FIG. 2 is an illustration of a hydraulic actuator in accordance with anillustrative embodiment;

FIG. 3 is an illustration of a hydraulic actuator in a static positionfor an on-ground condition, in accordance with an illustrativeembodiment;

FIG. 4 is an illustration of a hydraulic actuator in a lock-up positionin accordance with an illustrative embodiment;

FIG. 5 is an illustration of a hydraulic actuator in a fully extendedposition for stowing in accordance with an illustrative embodiment;

FIG. 6 is an illustration of a landing gear assembly in a staticposition in accordance with an illustrative embodiment;

FIG. 7 is an illustration of a landing gear assembly in a stow positionin accordance with an illustrative embodiment;

FIG. 8 is an illustration of a landing gear assembly in a landingposition in accordance with an illustrative embodiment;

FIG. 9 is an illustration of a block diagram of an aircraft inaccordance with an illustrative embodiment;

FIG. 10 is an illustration of a hydraulic strut assembly in the form ofa block diagram in accordance with an illustrative embodiment;

FIG. 11 is an illustration of a cross-sectional view of a hydraulicstrut assembly in accordance with an illustrative embodiment;

FIG. 12 is an illustration of an actuator in a compressed position inaccordance with an illustrative embodiment;

FIG. 13 is an illustration of an actuator in a retracted position inaccordance with an illustrative embodiment;

FIG. 14 is an illustration of an actuator in a fully extended positionin accordance with an illustrative embodiment;

FIG. 15 is an illustration of a landing gear assembly with an actuatorin a compressed position in accordance with an illustrative embodiment;

FIG. 16 is an illustration of a landing gear assembly with an actuatorin a retracted position in accordance with an illustrative embodiment;

FIG. 17 is an illustration of a landing gear assembly with an actuatorin a fully extended position in accordance with an illustrativeembodiment;

FIG. 18 is an illustration of a flowchart of a method of operating ahydraulic actuator in an aircraft, in accordance with an illustrativeembodiment;

FIG. 19 is an illustration of a process for operating a vehicle duringan alternate landing in the form of a flowchart in accordance with anillustrative embodiment;

FIG. 20 is an illustration of an aircraft manufacturing and servicemethod in accordance with an illustrative embodiment; and

FIG. 21 is an illustration of an aircraft in accordance with anillustrative embodiment.

DETAILED DESCRIPTION

The present disclosure now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred illustrativeembodiments of the disclosure are shown. This disclosure may, however,be embodied in many different forms and should not be construed aslimited to the illustrative embodiments set forth herein; rather, theseillustrative embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the disclosureto those skilled in the art. Like numbers refer to like elementsthroughout.

The illustrative embodiments recognize these issues and present asolution that is flexible, durable, relatively inexpensive compared toother struts, and lightweight. Additionally, the illustrativeembodiments have added further value to aircraft operation in that theillustrative embodiments aid an aircraft in both landing and lift off.The illustrative embodiments aid an aircraft to lift off by increasingthe angle of attack of the aircraft. The angle of attack is the angle atwhich an aircraft is attempting to lift off from the ground into theair. The illustrative embodiments aid an aircraft to land by providingadditional bogie beam pitch dampening. Other illustrative embodimentsare apparent from the following additional description.

Specifically, illustrative embodiments of the present disclosure relategenerally to landing gear assemblies and, more particularly, to asemi-levered landing gear assembly and an associated method ofpositioning the bogie beam of the landing gear assembly using atelescopic actuator. However, the illustrative embodiments may alsoapply to other vehicles and may be used in other applications aside fromvehicles. Thus, the illustrative embodiments are not limited to use inlanding gears or landing gear assemblies.

FIG. 1 is an illustration of a block diagram of an aircraft in which anillustrative embodiment may be implemented. While FIG. 1 may be used todescribe an aircraft incorporating the illustrative embodiments,aircraft 100 may also potentially be any other vehicle in which ahydraulic strut or hydraulic piston might be used.

Aircraft 100 includes fuselage 102, which is connected to wing 104. In anon-limiting illustrative embodiment, aircraft 100 may include engine106. In another illustrative embodiment, landing gear assembly 108 maybe connected to one of wing 104 or fuselage 102, or even possibly engine106, or possibly combinations thereof. Aircraft 100 may include manyother components. In an illustrative embodiment, landing gear assembly108 may include actuator 110 and other landing gear assembly components112.

Actuator 110 may include a nested series of hydraulic pistons sharingcommon outer wall 114. Thus, for example, actuator 110 may include firsthydraulic piston 116, second hydraulic piston 118, and third hydraulicpiston 120. In an illustrative embodiment, the three hydraulic pistonsare concentric. In an illustrative embodiment, the three hydraulicpistons may actuate in a telescopic manner such that, when fullyextended, second hydraulic piston 118 extends past a top of the thirdhydraulic piston 120, and second hydraulic piston 118 extends past a topof first hydraulic piston 116. Actuator 110 also includes manifold 122.Manifold 122 may be contained within common outer wall 114; however,manifold 122 may be connected in some other way to the first, second,and third hydraulic pistons. In any case, manifold 122 is disposedrelative to the first, second, and third hydraulic pistons (116, 118,and 120) such that a fluid moving in manifold 122 can control positionsof the first, second, and third hydraulic pistons (116, 118, and 120).Examples of such a fluid flow are detailed below with respect to FIGS. 2through 5.

Other arrangements are also possible. In other illustrative embodiments,one or more of the hydraulic pistons might be replaced by some otherkind of piston, such as an electromechanical piston.

In an illustrative embodiment, at least two of the first, second, andthird hydraulic pistons may share a common fluid source. In otherillustrative embodiments, all three hydraulic pistons share a commonfluid source. In an illustrative embodiment, more or fewer hydraulicpistons may be present. Thus, for example, four or more nested hydraulicpistons might be provided, though in another illustrative embodimentonly two nested hydraulic pistons might be provided.

In an illustrative embodiment, the different hydraulic pistons mighthave different operating pressures. Thus, for example, third hydraulicpiston 120 might maintain a constant pressure having a first value,whereas the second hydraulic piston 118 might maintain a constant returnpressure having a second value different than or the same as the firstvalue. However, pressures may vary; for example, the first hydraulicpiston 116 might be configured to operate at variable pressures betweenthird and fourth values different than the first and second values.Other combinations of operating pressures are possible.

The illustration of aircraft 100 in FIG. 1 is not meant to implyphysical or architectural limitations to the manner in which differentillustrative embodiments may be implemented. Other components inaddition to and/or in place of the ones illustrated may be used. Somecomponents may be unnecessary in some illustrative embodiments. Also,the blocks are presented to illustrate some functional components. Oneor more of these blocks may be combined and/or divided into differentblocks when implemented in different illustrative embodiments.

FIG. 2 is an illustration of a hydraulic actuator, in accordance with anillustrative embodiment. Hydraulic actuator assembly 200 shown in FIG. 2may be actuator 110 shown in FIG. 1. Likewise, other components maycorrespond between FIG. 1 and FIG. 2.

For example, first piston 206 may correspond to first hydraulic piston116, second piston 202 may correspond to second hydraulic piston 118,third piston 204 may correspond to third hydraulic piston 120, andcommon outer wall 214 may correspond to common outer wall 114. Hydraulicactuator assembly 200 may also be referred to as a telescopic hydraulicstrut assembly in some illustrative examples.

In the illustrative embodiment shown in FIG. 2, first piston 206, secondpiston 202, and third piston 204 are concentric to each other. Eachhydraulic piston has a corresponding pressure chamber. Thus, forexample, second piston 202 and third piston 204 share chamber 208, andfirst piston 206 has chamber 210. The space between common outer wall214 and first piston 206 define chamber 212. These chambers may operateat the same or different pressures, variable pressures, or a combinationof constant and variable pressures, all of which may be the same ordifferent.

In a non-limiting illustrative embodiment, the purpose of hydraulicactuator assembly 200 is to act as a fixed length tension member duringtakeoff, as shown in FIG. 8. In this configuration, hydraulic actuatorassembly 200 may be referred to as a hydraulic strut. During takeoffroll, the load on the landing gear assembly is reduced as the wingsgenerate lift. The reduced load on the landing gear shock absorber 604may cause the lower portion 802 of the shock absorber 604 to extend suchthat the bogie beam 602 is forced to pivot about upper lug pivot 612rather than about the main pivot 616 so as to provide a semi-leveredfunction to the landing gear assembly 600. As a result, the aircraft mayexperience a greater ground clearance, which in turn allows the airplaneto rotate to a greater angle of attack on takeoff.

In a non-limiting illustrative embodiment, to perform the semi-leveredfunction of a hydraulic actuator, chamber 212 is filled with fluid to anillustrative pressure greater than the fluid pressure in chamber 210.This result is shown in FIGS. 3 and 4. The greater fluid pressure inchamber 212 causes the first piston 206 to be fully retracted inside thecylinder barrel 215. FIG. 3 shows the on-ground configuration where thefirst piston 206 is fully retracted, but the second piston 202 and thethird piston 204 may move, allowing fluid to pass in and out of chambers210 and 208. This movement of fluid in and out of chambers 210 and 208provides dampening, which is an illustrative function to resist bogiebeam pitch about main pivot 616 of FIGS. 6 through 8.

During takeoff roll, the load on the landing gear assembly is reduced asthe wings generate lift. The reduced load on the landing gear shockabsorber causes the lower portion of the shock absorber 604 to extend.The extending motion of the shock absorber causes the hydraulic actuatorassembly 200 to extend to the position shown in FIG. 4. In thisposition, the second piston 202 is pulled against stops on the end offirst piston 206. This position achieves the semi-lever functionality ofthe hydraulic actuator and landing gear assembly.

Referring to FIG. 3 in conjunction with the above description of FIG. 2,in this illustrative embodiment, the hydraulic actuator assembly 200passively transitions from position 300 to position 400 in response tothe loads applied to the aircraft and landing gear assembly. Thistransition may not require any input from the pilots, crew, or any othermechanical or electrical device to achieve this desirable functionality.This passive operation reduces mechanical and hydraulic complexity andincreases reliability.

Hydraulic actuator assembly 200 may have other functions. For example,hydraulic actuator assembly 200 may aid in positioning the bogie beam602 of FIGS. 6 through 8, to different positions of varying lengths,such as stow or landing positions. In typical large aircraftconfigurations, it is beneficial to position the bogie beam 602 of FIG.7 in an attitude where the forward axle is lower than the aft axle forstorage in a wheel well. In this instance, the hydraulic actuatorassembly 200 can be lengthened to position 500 as shown in FIG. 5. Thisposition is achieved by decreasing the fluid pressure in chamber 212,which allows the pressure in chamber 208 to extend the hydraulicactuator assembly 200. In this manner, passages in the manifold allowthe fluid in chamber 212 to exit the chamber. In some instances it maybe beneficial to integrate the command to assume position 500 with thelanding gear assembly retraction command such that the hydraulicactuator commands position 500 automatically when the pilot commands thelanding gear assembly to be retracted.

Hydraulic actuator assembly 200 may allow for extension during landingtouchdown to allow a change in bogie beam pitch to facilitate air-groundsensing. In particular, hydraulic actuator assembly 200 may allow forextension during landing touchdown to allow a change in bogie beam pitchto provide even tire loading. Hydraulic actuator assembly 200 mayprovide damping during landing to limit loads into the other parts ofthe aircraft. Hydraulic actuator assembly 200 may provide bogie beampitch damping, as shown further in FIG. 6.

Returning to FIG. 2, the second piston 202 may operate with a constantpressure, such as about 2000 pounds per square inch (psi) in onenon-limiting illustrative embodiment (possibly more or fewer psi) bypressuring the fluid in chamber 208 accordingly. The constant pressuremay be selected to provide sufficient force to position a bogie beam tostow, while not producing excessive force while on the ground, whichcould undesirably load the tires.

In an illustrative embodiment, the third piston 204 may maintain aconstant downward force due to pressure in chamber 208 being greaterthan chamber 210. This force may reduce the extend forces and reduce theareas that experience system pressure.

In an illustrative embodiment, the first piston 206 may operate atvariable pressures by varying the pressure of the fluid in chamber 212.The pressure in chamber 212 may be varied depending on the mode ofoperation of the hydraulic actuator assembly 200. For example, arelatively low pressure of about 500 psi may be used in chamber 212 forlanding to allow the bogie beam to move for air-ground sensing, thoughhigher or lower pressure might be used for this purpose depending on theaircraft and design considerations. On the other hand, chamber 212 mayoperate at about 3000 to about 5000 psi, or greater, in order to lockthe hydraulic actuator assembly 200. In this case, the hydraulicactuator assembly 200 may act as a tension member during lift offrotation of the aircraft. Later, a reduced pressure of system return inchamber 212 may cause the strut to telescopically extend the nestedhydraulic pistons 206, 204, and 202 while bringing the strut and bogiebeam to a stow position.

In an illustrative embodiment, the second piston 202 may be referred toas a main piston, the first piston 206 may be referred to as atelescopic piston, and the third piston 204 may be referred to as afloating piston. In an illustrative embodiment, third piston 204 andguide tube 238 may define chamber 239, which is common with chamber 208and which may greatly reduce the hydraulic flow used to reposition thehydraulic actuator assembly 200. As a result, the time used to extendthe hydraulic actuator assembly 200 for stowing in the wheel well may beillustratively reduced since the flow into chamber 208 from systemsupply 250 is much less than if chamber 210 had to be filled usingsystem supply 250.

Attention is now turned to pressure ranges with respect to hydraulicactuator assembly 200. In the illustrative embodiment shown, pressureranges are for a system operating between about 500 psi and 5000 psi,though other ranges might be suitable and could vary by as much as about0 psi to about 10,000 psi or more. These pressures are approximate andmay vary with each specific operation or implementation. Seals are notshown, but conventional seals may be used in each groove shown inhydraulic actuator assembly 200.

In an illustrative embodiment, multi-mode reducer 216 may provide threeoutlet pressures using a single valve, as shown. These pressures mightbe 0 psi, 500 psi, and 5000 psi, as indicated at dashed sensing line218. The single valve may provide three outlet pressures by using astandard pressure reducer and adding solenoid valve input 220 andsolenoid valve input 222 to either end as shown. Solenoid valve input220 and solenoid valve input 222 may be actuated to drive the valve tobe fully on or fully off. When solenoid valve input 220 is on, then thepressure may be about 0 psi. When solenoid valve input 222 is on, thenthe pressure may be about 5000 psi. When both solenoid valve input 220and solenoid valve input 222 are off, multi-mode reducer 216 may performas a normal reducer, outputting about 500 psi in this example. The about500 psi may be low enough to hold the bogie beam in a landing attitudebut still allow the bogie beam to move at touchdown, allowing theaircraft to use initial bogie beam motion to trigger landing spoilers.In particular, pressure of about 500 psi may be low enough to hold thebogie beam in a landing attitude but still allow the bogie beam to moveat touchdown, allowing the tires to be evenly loaded when the aircraftcontacts the ground.

Multi-mode relief valve 224 may be an adaptation of a common reliefvalve with solenoid valve inputs, which may be the same valve inputsused in multi-mode reducer 216. Thus, for example, solenoid valve input226 may cause the relief valve to be opened, for use in the stowposition, and solenoid valve input 228 may be used to put the reliefvalve into its high pressure setting. The solenoid valve input 228 mayincrease the cracking pressure from about 1000 psi to about 5500 psi byincreasing the spring pre-load. A use for the multi-mode relief valve224 may be to provide touchdown damping in order to reduce loads in thefuselage and other parts of the airframe, which saves weight. Duringtouchdown, the first piston 206 and the second piston 202 may be pulledout rapidly. Fluid from the rod end of chamber 212 may exit through themulti-mode relief valve 224, which may be sized to provide the properdamping rate.

A pressure sensor 240 may be used to verify that the hydraulic actuatorassembly 200 is locked. If the pressure sensor senses that pressure isnear maximum system pressure, then the hydraulic actuator assembly 200may react to the full tension load expected during takeoff with asemi-levered landing gear. Note that if the seals are damaged, fullpressure would not be achieved and/or sensed by pressure sensor 240,thereby providing an illustrative method of testing the integrity of thehydraulic actuator assembly 200.

Check valve 230 may be a check valve that may trap fluid in hydraulicactuator assembly 200 in order to hold the hydraulic actuator assembly200 in the fully extended position. In an illustrative embodiment, thehydraulic pressure may be removed from system supply 250 after thelanding gear is retracted, and check valve 230 also holds the bogie beamin position while the landing gear is tucked into the wheel well.

Reducer 232 may provide reduced pressure to the chamber 208. Thisreduced pressure may be selected so as to avoid overloading the fronttires while the aircraft is on the ground, but being sufficient pressureto power the actuator to the fully extended position when gear up isselected. A possible alternative illustrative embodiment may be toprovide a solenoid input to reducer 232 in order to shut reducer 232 offwhile the aircraft is on the ground. In this illustrative embodiment,the tires may be equally loaded.

Check valve 234 may be used in an alternate extension case, such aswhere the landing gear assembly is extended by alternate means afterhydraulic system loss. This use may leave the hydraulic actuatorassembly 200 fully extended so that the aircraft may land with fronttires down. This landing procedure may cause a rapid compression of thehydraulic actuator assembly 200. The second piston 202 may move first,which may force fluid out of chamber 210 and back towards reducer 232.In this case, the fluid in chamber 208 may also flow to system return242. In an illustrative embodiment, accumulator 248 may be provided forsurge suppression.

In any case, relief valve 236 may allow the fluid in chamber 208 to flowinto chamber 212, forcing the first piston 206 down. This action startsthe first piston 206 moving before the second piston 202 reaches thefirst piston 206, which reduces impact loads. If the fluid flow fromchamber 210 exceeds the return line capacity, then that flow may flowthrough the check valve 234 to chamber 212, further aiding the motion ofthe first piston 206. When the second piston 202 reaches the firstpiston 206, the second piston 202 may contact stop 244.

In an illustrative embodiment, the third piston 204 may be containedwithin the second piston 202, in which case, guide tube 238 may extendfrom the head end of cylinder barrel 215. In this case, the third piston204 may have a stop 246 that prevents the third piston 204 fromdeparting from guide tube 238 if the third piston 204 attempts toover-extend.

Thus, FIG. 2 depicts one illustrative embodiment of hydraulic strut 606of FIGS. 6 through 8 in greater detail. Hydraulic actuator assembly 200includes a cylinder barrel 215, a first piston 206 slidably receivedthrough an open end of the cylinder barrel 215, and second piston 202slidably received through an open end of first piston 206. The secondpiston 202 may include at least one lug or other connecting member atits upper end for attachment to the landing gear assembly upper half, asshown in FIGS. 6 through 8. The cylinder barrel 215 may include at leastone lug or other connecting member at its lower end for attachment tobogie beam 602 at upper lug pivot 612, both of FIGS. 6 through 8. Thecylinder barrel 215 also contains a guide tube 238 that is fixed to thecylinder barrel 215. A floating piston, third piston 204, is containedwithin the second piston 202 and the guide tube 238. The upper end ofcylinder barrel 215 sealingly engages with the outer surface of firstpiston 206. The lower end of the first piston 206 sealingly engages withthe inner surface of the cylinder barrel 215.

The cylinder barrel 215 includes fluid passages as shown in FIG. 2 tosupply chambers 212, 210, and 208 with pressurized fluid. These passagesand chambers constitute a manifold contained within the common outerwall, the manifold is disposed relative to the first, second, and thirdhydraulic pistons such that a fluid moving in the manifold can controlpositions of the first, second, and third hydraulic pistons. Thefeatures of the hydraulic manifold shown in FIG. 2 allow the pressuresin chamber 212 to be changed such that the first piston 206 may beforced in or out of the cylinder barrel 215 in a desirable manner. Notethat the manifold may take other forms. For example, the manifold may bea series of possibly different (more or fewer than those shown) chambersconnected in some other way to the first, second, and third hydraulicpistons. In any case, the manifold is disposed relative to the first,second, and third hydraulic pistons such that a fluid moving in themanifold can control positions of the first, second, and third hydraulicpistons.

The upper end inside surface of first piston 206 sealingly engages withthe outer surface of the second piston 202. The inside surface of secondpiston 202 sealingly engages with the upper outside surface of thirdpiston 204. The upper end inside surface of the guide tube 238 sealinglyengages with the outer surface of the third piston 204. The cylinderbarrel 215 includes fluid passages as shown in FIG. 2 to supply chambers208, 210, and 212 with pressurized fluid. The features of the hydraulicactuator assembly 200 shown in FIG. 2 allow the pressures in chambers208 and 212 to be changed such that the second piston may be forced outof the first piston 206 in a desirable manner and both second piston 202and third piston 204 can be extended together.

As implied above, the nested pistons shown in hydraulic actuatorassembly 200 may have different arrangements to achieve differentfunctions. Furthermore, different valves, reducers, and other hydrauliccomponents may be arranged to change how hydraulic fluids flow withinthe various fluid chambers of hydraulic actuator assembly 200, again toachieve different functions. Thus, the illustrative embodiments are notlimited by the particular arrangements described with respect to FIG. 2.

FIG. 3 through FIG. 5 are illustrations of a hydraulic actuator in use,in accordance with an illustrative embodiment. The illustrativeembodiments shown in FIG. 3 through FIG. 5 correspond to the hydraulicactuator assembly 200 shown in FIG. 2. Therefore, reference numerals inFIG. 3 through FIG. 5 sharing the same value as the reference numeralsin FIG. 2 may correspond to the same components and may have similarstructure and functions. Not all components described with respect toFIG. 2 are necessarily shown with respect to FIGS. 3 through 5; however,all such components may be present in some illustrative embodiments.

The illustrative embodiments shown in FIG. 3 through FIG. 5 showhydraulic actuator assembly 200 in use. In FIG. 3, the hydraulicactuator assembly 200 has a position 300 for use while the aircraft ison the ground. In FIG. 4, the hydraulic actuator assembly 200 has aposition 400. In FIG. 5, the hydraulic actuator assembly 200 has aposition 500.

In the illustrative embodiment shown in position 300, the chamber 208may have a pressure of about 2000 psi, but that value may be more orless. The chamber 210 is at the return pressure, which may be a constantpressure of about 50 psi. The chamber 212 may have a pressure of about500 psi. In this arrangement, third piston 204 and first piston 206 areheld down by pressure in chambers 208 and 212. Second piston 202 slidesaxially as the bogie beam moves.

This position 300 of hydraulic actuator assembly 200 may be beneficialwhen the airplane is on the ground. The position may be beneficialbecause the hydraulic actuator assembly 200 allows for normal bogie beampitch motion without excessive loads in the hydraulic actuator.Furthermore, the hydraulic actuator may be arranged so as to avoidimpact against a lockup position to avoid overloading the front tires.Additionally, the hydraulic actuator may be short enough to preventoverload of the hydraulic actuator in the event of an unexpectedcondition such as one or more tires on the aft landing gear axlesexperiencing lowered air pressure.

In position 400, the pressure in chamber 208 and chamber 210 ismaintained, but the pressure in chamber 212 may be increased so as torestrain first piston 206 in a fully compressed position. Position 400is illustrative during takeoff. Position 400 is beneficial on takeoffbecause hydraulic actuator assembly 200 has a fixed length, which hasthe effect of pulling up on the front of the bogie beam as the landinggear shock absorber pushes down, which causes the rear tires to beforced down. As a result, the effective length of the landing gearassembly is longer at the point of rotation, which allows the airplaneto rotate to a higher angle of attack.

During landing, position 400 causes hydraulic actuator assembly 200 tosee an initial tension load. In this manner, position 400 may act as adamper during initial touchdown as fluid is forced out of chamber 212.

In position 500, the pressure in chamber 212 is removed so that thepressure in chamber 208 will fully extend second piston 202. Theextension of second piston 202 will pull third piston 204 to its fullextended position. As a result, hydraulic actuator assembly 200 reachesmaximum telescopic extension of each of the three hydraulic pistons suchthat the top of second piston 202 extends past the top of third piston204. Position 500 is illustrative because this position orients thebogie beam in the desirable attitude to fit inside the wheel well. Nosupply pressure is present and no single issue or change in thehydraulic actuator assembly configuration can cause large retractionforces.

FIGS. 6 through 8 illustrate a landing gear assembly in three differentpositions in several illustrative embodiments. FIG. 6 illustrates alanding gear assembly 600 in the ground position; FIG. 7 illustrateslanding gear assembly 600 in the stow position; and FIG. 8 illustrateslanding gear assembly 600 in a landing position. Reference numerals inFIGS. 6 through 8 sharing the same value as the reference numerals maycorrespond to similar components and may have similar structure andfunctions. In one possible non-limiting illustrative embodiment, thesame components among FIGS. 6 through 8 may be the same and have thesame functions. The illustrative embodiments shown in FIGS. 6 through 8are non-limiting examples of one possible use of hydraulic actuatorassembly 200 shown in FIGS. 2 through 5. A possible operation of landinggear assembly 600 in conjunction with hydraulic strut 606 is describedwith respect to FIGS. 2 through 5.

Turning first to FIG. 7, an illustration of a landing gear assembly inthe stow position is shown, in accordance with an illustrativeembodiment. Landing gear assembly 600 includes hydraulic strut 606.Hydraulic strut 606 may be the same or similar to hydraulic actuatorassembly 200 shown in FIG. 2 through FIG. 5. The illustrative embodimentshown in FIG. 7 is a non-limiting example of one possible use ofhydraulic actuator assembly 200 shown in FIGS. 2 through 5. A possibleoperation of landing gear assembly 600 in conjunction with hydraulicstrut 606 is described with respect to FIGS. 2 through 5.

Turning now to FIG. 6, hydraulic strut 606 is shown in the groundconfiguration, which may correspond to position 300 shown in FIG. 3.Landing gear assembly 600 also shows other features, some of which aredescribed above with respect to FIGS. 2 through 5. These featuresinclude bogie beam 602 attached to the lower portion of shock absorber604. Lug 608 is attached to the cylinder portion of shock absorber 604.Plurality of wheels 610 are attached to bogie beam 602. Plurality ofwheels 610 may include forward wheels 610B and aft wheels 610A.Hydraulic strut 606 is pivotally attached to the upper portion of shockabsorber 604 at lug 608. Hydraulic strut 606 is pivotally attached tobogie beam 602 at lower lug pivot 612. Shock absorber 604 is attached tobogie beam 602 by main pivot 616. In use, lug 608 and lower lug pivot612 allow hydraulic strut 606 to move in two different orientations withrespect to shock absorber 604 and bogie beam 602. In use, main pivot 616allows the ends of bogie beam 602 to pivot upwardly and downwardly withrespect to shock absorber 604.

FIG. 7 also depicts the hydraulic strut 606 with the second piston 700(corresponding to second piston 202 of FIG. 2) pivotally attached to theupper portion of the shock absorber 604 via lug 608. The cylinder barrel607 (corresponding to cylinder barrel 215 of FIG. 2) of hydraulic strut606 is pivotally attached to the bogie beam at the lower lug pivot 612.In other illustrative embodiments, the hydraulic strut 606 may bereoriented such that the second piston (700/202) may be attached to thelower lug pivot 612 to the bogie beam 602 and the cylinder barrel(215/607) may be attached to the cylinder portion of the shock absorber604.

As shown in FIG. 7, hydraulic strut 606 is actuated such that secondpiston (700/202) and telescopic, first piston (702/206) are extended. Inan embodiment, both are fully extended. In this orientation, one end ofbogie beam 602 is forced downwardly about main pivot 616. Thisorientation and operation is described further with respect to FIGS. 2through 5.

After liftoff, the hydraulic strut 606 positions the landing gearassembly 600 at an angle, such that the forward axle is lower than theaft axle, as shown in FIG. 7. In an illustrative embodiment, the anglemay be twelve degrees, though this value may be varied between less thana degree to eighty degrees or more. Hydraulic strut 606 may berepositioned quickly to stow position shown in FIG. 7 using the smallflow required to fill chamber 208 of FIG. 2.

Later, the hydraulic strut 606 may be hydraulically de-energized. Whilein the wheel well, the hydraulic strut 606 may maintain the fullyextended position with no supply pressure. The return pressure inchamber 210 may aid in this function. While in this position, no singlefailure may cause large retraction forces.

Turning now to FIG. 8, the hydraulic strut 606 is depicted with thesecond piston 700 (corresponding to second piston 202 of FIG. 2)pivotally attached to the upper portion of the shock absorber 604 vialug 608. The cylinder barrel 607 (corresponding to cylinder barrel 215of FIG. 2) of hydraulic strut 606 is pivotally attached to the lower lugpivot 612 which is attached to the bogie beam. In other illustrativeembodiments, the hydraulic strut 606 may be reoriented such that thesecond piston (700/202) may be attached to the lower lug pivot 612 tothe bogie beam 602 and the cylinder barrel (215/607) may be attached tothe cylinder portion of the shock absorber 604 at lug 608.

In an illustrative embodiment, the bogie beam angle with respect to theground may be 23 degrees, though this value may be varied to suit therequirements of the vehicle. This orientation and operation is describedfurther below, with respect to FIGS. 2 through 5.

Before landing, the hydraulic strut 606 positions the landing gearassembly from position 500 (FIG. 5) to position 400 (FIG. 4) byretracting the first piston 206. This position tilts the bogie beam 602for a landing position such that the forward axle is higher than the aftaxle. In this position, the hydraulic strut 606 is restrained with aprescribed amount of force by pressure in chamber 212 of FIG. 2.

During landing, the aft tires will contact the ground first, causing thebogie beam to rotate about main pivot 616. This motion may causehydraulic strut 606 to experience an initial high tension load.Hydraulic strut 606 may move with initial low resistance to allow anair-ground sensing system to detect the change in pitch of the bogiebeam. As the shock absorber 604 compresses, the bogie beam will continueto rotate about main pivot 616 until the forward tires contact theground. Once the forward tires touch the ground, the hydraulic strut 606may experience rapid compression. Hydraulic strut 606 may act as adamper during initial touchdown. In an illustrative embodiment,hydraulic strut 606 may allow the aircraft to land when hydraulic strut606 is in a fully extended position as shown in FIG. 7, if no hydraulicpressure is available, in order to provide for an alternate landingposition.

While on the ground, the hydraulic strut 606 allows for normal pitchmotion of bogie beam 602 around a main pivot 616 without excessive loadsin the hydraulic strut 606 and without overloading the tires. In anillustrative embodiment, hydraulic strut 606 may collapse short enoughto prevent any unexpected conditions from impairing landing gearassembly 600 or the aircraft.

Considering FIGS. 6 through 8 together, a semi-levered landing gearassembly 600 in accordance with an illustrative embodiment of thedisclosure is shown. The landing gear assembly 600 includes a shockabsorber 604 of suitable construction to absorb and dampen transientloads exerted between the gear and the ground during ground operationsof an aircraft, and to support the aircraft when stationary on theground. The shock absorber 604 typically includes an upper portion 800and a lower portion 802 that is telescopingly received in the upperportion such that the length of the shock absorber 604 can varydepending on the amount of the load applied to the landing gear assemblyin a direction along the axis of the shock absorber. On initialtouchdown, as shown in FIG. 8, the amount of load applied to the landinggear assembly 600 is relatively small and, accordingly, the length ofthe shock absorber 604 is about at a maximum.

The landing gear assembly 600 further includes a wheel truck 804 formedby at least one bogie beam 602 pivotally attached at main pivot 616 to alower portion 802 of the shock absorber 604. A plurality of wheels 610are rotatably supported by the bogie beam 602, including at least oneforward wheel and at least one aft wheel respectively supported at aforward end and an aft end of the bogie beam 602. In general, for mostlarge aircraft, the wheel truck of a main landing gear assembly mayinclude a plurality of wheels 610, which may include a pair of forwardwheels on an axle at the forward end of bogie beam 602 and a pair of aftwheels on an axle at the aft end of bogie beam 602. Some illustrativeembodiments may include a plurality of wheels on one or more additionalaxles between the forward and aft axles. However, the illustrativeembodiments described herein are applicable to any wheel truckconfiguration having at least one wheel supported by a bogie beam at alocation that is longitudinally displaced forward of a main pivot and atleast one wheel supported by a bogie beam at a location that islongitudinally displaced aft of a main pivot.

The landing gear assembly 600 also includes a hydraulic strut 606, whichmay be hydraulic actuator assembly 200 of FIG. 2. Hydraulic strut 606 ispivotally connected at its upper end to the lug 608 at the shockabsorber 604 and has its lower end pivotally connected at lower lugpivot 612 on the bogie beam 602 at a location forward of main pivot 616.The hydraulic strut 606 is a variable-length device enabling the bogiebeam 602 to pivot relative to the shock absorber 604. Additionally, thehydraulic strut 606 is capable of locking up at a fixed length, whensuitably controlled as further described above, such that the bogie beam602 is forced to pivot about lower lug pivot 612 rather than about mainpivot 616, so as to provide a semi-levered function to the landing gearassembly 600.

FIG. 9 is an illustration of a block diagram of an aircraft, inaccordance with an illustrative embodiment. Aircraft 900 shown in FIG. 9may be, for example, aircraft 100 shown in FIG. 1. The variouscomponents described with respect to FIG. 9 may also be found in FIGS. 2through 8, as described further below.

Aircraft 900 includes landing gear 902, which may include a plurality ofaxles 904 upon which a plurality of tires 905 are disposed. Landing gear902 may have, in other embodiments, one or more axles including one ormore tires. Landing gear 902 may be, in some embodiments, landing gearassembly 108 of FIG. 1 or landing gear assembly 600 of FIGS. 6 through8. Plurality of axles 904 may be, for example, part of bogie beam 602 ofFIGS. 6 through 8. Plurality of tires 905 may be, for example, pluralityof wheels 610 of FIGS. 6 through 8.

Landing gear 902 may also include manifold 906. An actuator 910 isdisposed within manifold 906. In an illustrative embodiment, thepressure of fluid 908 may be varied and then applied to the actuator 910such that landing gear 902 is restrained in a landing position byactuator 910 with a force that is suitably low to also allow forair-ground sensing during touchdown of the aircraft 900.

Fluid 908 may be, for example, the fluid that flows through a manifolddisposed with respect to manifold 906. In a particular example, fluid908 may flow within chambers, such as chambers 208, 210, and 212 ofFIGS. 2 through 5. Actuator 910 may take other forms, as well, such asadditional pistons in a nested piston arrangement.

In an embodiment, manifold 906 may include multi-mode reducer valve 912.Multi-mode reducer valve 912 may be, for example, multi-mode reducer 216of FIG. 2. Multi-mode reducer valve 912 may be configured to allowvariable pressure settings for fluid.

In an embodiment, manifold 906 may include multi-mode relief valve 914.Multi-mode relief valve 914 may be, for example, multi-mode relief valve224 of FIG. 2. Multi-mode relief valve 914 may be configured to allowfluid 908 to exit manifold 906. In another embodiment, multi-mode reliefvalve 914 may be configured to reduce a pressure of fluid 908 whileaircraft 900 is on the ground in order to balance loads among theplurality of axles 904.

In an embodiment, an accumulator 916 may be disposed with respect tomanifold 906 such that accumulator 916 absorbs pressure spikes duringtouchdown of the aircraft 900. Accumulator 916 may be, for example,accumulator 248 of FIG. 2.

In an embodiment, pressure sensor 918 may be connected to at least oneof manifold 906 and actuator 910. Pressure sensor 918 may be configuredto monitor a health of landing gear 902. Pressure sensor 918 may be, forexample, pressure sensor 240 of FIG. 2.

The illustration of aircraft 900 in FIG. 9 is not meant to implyphysical or architectural limitations to the manner in which differentillustrative embodiments may be implemented. Other components inaddition to and/or in place of the ones illustrated may be used. Somecomponents may be unnecessary in some illustrative embodiments. Also,the blocks are presented to illustrate some functional components. Oneor more of these blocks may be combined and/or divided into differentblocks when implemented in different illustrative embodiments.

The different illustrative embodiments recognize and take into accountthat actuator 110 in FIG. 1, hydraulic actuator assembly 200 in FIGS. 2through 5, hydraulic strut 606 in FIGS. 6 through 8, actuator 910 inFIG. 9, manifold 906 in FIG. 9, are examples of differentimplementations for a hydraulic actuator for a landing gear assembly.The different illustrative embodiments recognize and take into accountthat these hydraulic actuators use hydraulic fluid.

For example, the fluid flowing through manifold 122 in FIG. 1 and fluid908 flowing through manifold 906 in FIG. 9 is hydraulic fluid. Thishydraulic fluid may be, for example, without limitation, a liquidcomprising, for example, without limitation, phosphate-ester hydraulicfluid.

The different illustrative embodiments recognize and take into accountthat the hydraulic actuator may have an alternate extension state whenthe aircraft performs an alternate landing. As used herein, an“alternate landing” is a landing performed when hydraulic system poweris unavailable to control the landing gear for the aircraft. Forexample, an alternate landing may be an emergency landing.

During an alternate landing, a landing gear assembly for an aircraft maybe configured with the wheeled truck assembly positioned with theforward axle lower than the aft axle. Consequently, the hydraulicactuator in the alternate extension state may need to compress morerapidly during an alternate landing as compared to a typical landing.

The different illustrative embodiments recognize and take into accountthat rapid compression of the hydraulic actuator may require that thehydraulic fluid be expelled from the hydraulic actuator. In some cases,this expulsion of hydraulic fluid from the hydraulic actuator mayrequire higher flow rates than desired. Further, in some cases, managingthese high flow rates may be more difficult and require larger and/orheavier components than desired.

The different illustrative embodiments recognize and take into accountthat a different configuration for the pistons used in actuator 110 inFIG. 1, hydraulic actuator assembly 200 in FIGS. 2 through 5, hydraulicstrut 606 in FIGS. 6 through 8, and actuator 910 in FIG. 9 and/orintroducing a compressible gas into these hydraulic actuators may allowthese hydraulic actuators to rapidly compress during an alternatelanding. In particular, using a compressible gas in a hydraulic actuatormay reduce the amount of hydraulic fluid that needs to be expelled fromthe hydraulic actuator.

Thus, the different illustrative embodiments provide a hydraulicactuator configured to also use a compressible gas that does not need tobe expelled from the hydraulic actuator when the hydraulic actuator israpidly compressed during an alternate landing. In one illustrativeembodiment, a hydraulic strut assembly comprises a housing, a firstpiston, a second piston, and a third piston. The housing comprises outerand inner cylindrical structures. An outer chamber is formed between theouter cylindrical structure and the inner cylindrical structure and isconfigured to receive a first fluid. The first piston is positionedbetween the outer and inner cylindrical structures. The second piston isnested within the first piston. The inner cylindrical structure, thefirst piston, and the second piston form an inner chamber in which avolume of the inner chamber changes when at least one of the first andsecond pistons move. The inner chamber is configured to hold a secondfluid comprising a gas. The third piston is positioned between the outercylindrical structure and the first piston. The first, second, and thirdpistons are configured to move in a direction parallel to an axisthrough the housing.

Turning now to FIG. 10, an illustration of a hydraulic strut assembly inthe form of a block diagram is depicted in accordance with anillustrative embodiment. In these illustrative examples, hydraulic strutassembly 1000 may be part of landing gear assembly 1002.

Hydraulic strut assembly 1000 may also be referred to as a strutassembly or a telescopic hydraulic strut assembly. Further, in somecases, hydraulic strut assembly 1000 may be referred to as an actuatorassembly or a hydraulic actuator assembly.

Landing gear assembly 1002 is an example of one implementation forlanding gear assembly 108 in FIG. 1. Landing gear assembly 1002 is asemi-levered landing gear assembly in these illustrative examples. Asdepicted, landing gear assembly 1002 may be part of aircraft 1004 inthese examples. In other illustrative examples, landing gear assembly1002 may be part of some other suitable type of aerospace vehicle.

Landing gear assembly 1002 may take the form of any assembly configuredto enable semi-levered action. As depicted, hydraulic strut assembly1000 in landing gear assembly 1002 comprises actuator 1001 and manifold1005. In some illustrative examples, landing gear assembly 108 inaircraft 100 in FIG. 1 may use hydraulic strut assembly 1000 in FIG. 10instead of actuator 110 in FIG. 1. Further, in other illustrativeexamples, landing gear 902 in FIG. 9 may use actuator 1001 and manifold1005 in FIG. 10 instead of actuator 910 and manifold 906 in FIG. 9.

As depicted, actuator 1001 comprises housing 1006, first piston 1008,second piston 1010, and third piston 1012. First piston 1008, secondpiston 1010, and third piston 1012 may be referred to as hydraulicpistons in some illustrative examples. In other illustrative examples,first piston 1008, second piston 1010, and third piston 1012 may bereferred to as a telescopic piston, a main piston, and a floatingpiston, respectively.

In these illustrative examples, housing 1006 comprises outer cylindricalstructure 1014 and inner cylindrical structure 1016. Outer cylindricalstructure 1014 may be formed by an outer wall having an inner surfaceand an outer surface. Inner cylindrical structure 1016 may be formed byan inner wall having an inner surface and an outer surface.

Inner cylindrical structure 1016 is located within outer cylindricalstructure 1014. Further, inner cylindrical structure 1016 may beassociated with outer cylindrical structure 1014. For example, housing1006 has first end 1020 and second end 1022. First end 1020 may be thebottom end of housing 1006, while second end 1022 may be the top end ofhousing 1006. Inner cylindrical structure 1016 may be associated withouter cylindrical structure 1014 at second end 1022 of housing 1006.

Further, axis 1024 is an axis that runs through housing 1006 from firstend 1020 of housing 1006 to second end 1022 of housing 1006. In oneillustrative example, axis 1024 is a center axis through actuator 1001.For example, axis 1024 may be a center axis along which both innercylindrical structure 1016 and outer cylindrical structure 1014 arealigned. In this manner, inner cylindrical structure 1016 and outercylindrical structure 1014 may be concentric to each other with respectto axis 1024. Movement in a direction parallel to axis 1024 may beconsidered linear movement.

First piston 1008, second piston 1010, and third piston 1012 areassociated with housing 1006. When one component is “associated” withanother component, the association is a physical association in theseillustrative examples. For example, a first component, such as firstpiston 1008, may be considered to be associated with a second component,such as housing 1006, by being secured to the second component, bondedto the second component, mounted to the second component, welded to thesecond component, fastened to the second component, and/or connected tothe second component in some other suitable manner. Further, the firstcomponent may be movably connected to the second component such that atleast one of these components may move relative to the other component.

Further, the first component also may be connected to the secondcomponent using a third component. The first component may also beconsidered to be associated with the second component by being formed aspart of and/or an extension of the second component. For example, thirdpiston 1012 is used to associate first piston 1008 with housing 1006.Further, first piston 1008 is used to associate second piston 1010 withhousing 1006.

Additionally, the first component may be considered to be associatedwith the second component by being physically connected to the secondcomponent in a manner that physically constrains motion of the firstcomponent relative to the second component. For example, first piston1008 may be associated with housing 1006 in a manner that causes motionof first piston 1008 to be constrained relative to housing 1006. Themovement of first piston 1008 may be constrained to movementsubstantially parallel to axis 1024.

In particular, first piston 1008, second piston 1010, and third piston1012 are a nested series of pistons. In these illustrative examples,these three pistons are concentric to each other with respect to axis1024. In particular, first piston 1008 may be disposed within thirdpiston 1012 and second piston 1010 may be disposed within first piston1008. In this manner, first piston 1008, second piston 1010, and thirdpiston 1012 may be substantially aligned with respect to axis 1024 inthese illustrative examples.

First piston 1008 is positioned between outer cylindrical structure 1014and inner cylindrical structure 1016. In particular, first piston 1008is located between an inner surface of outer cylindrical structure 1014and an outer surface of inner cylindrical structure 1016. Second piston1010 is nested within first piston 1008.

As depicted, first piston 1008 has first end 1026 and second end 1028.First end 1026 may be the top end of first piston 1008, while second end1028 may be the bottom end of first piston 1008. Further, second piston1010 has first end 1030 and second end 1032. First end 1030 may be thetop end of second piston 1010, while second end 1032 may be the bottomend of second piston 1010.

In these illustrative examples, first piston 1008 is configured to movein a direction parallel to axis 1024 relative to first end 1020 ofhousing 1006. In other words, first piston 1008 may move in a directionparallel to axis 1024 such that a position of first piston 1008 relativeto first end 1020 of housing 1006 changes.

For example, a position of second end 1028 of first piston 1008 relativeto first end 1022 of housing 1006 changes when first piston 1008 movesin a direction parallel to axis 1024. When first piston 1008 moves in adirection towards second end 1022 of housing 1006, first piston 1008 isconsidered to be retracting. When first piston 1008 moves in a directionaway from second end 1022 of housing 1006, first piston 1008 isconsidered to be extending.

In these illustrative examples, second piston 1010 is configured to movein a direction parallel to axis 1024 relative to second end 1028 offirst piston 1008. In other words, second piston 1010 may move in adirection parallel to axis 1024 such that a position of second piston1010 relative to second end 1028 of first piston 1008 changes.

For example, the position of first end 1030 of second piston 1010relative to second end 1028 of first piston 1008 changes when secondpiston 1010 moves in a direction parallel to axis 1024. When secondpiston 1010 moves in a direction towards second end 1022 of housing1006, second piston 1010 is considered to be retracting. When secondpiston 1010 moves in a direction away from second end 1022 of housing1006, second piston 1010 is considered to be extending.

When actuator 1001 is part of landing gear assembly 1002, second end1032 of second piston 1010 may be connected to beam 1034 in landing gearassembly 1002. Beam 1034 may be referred to as a “truck beam” or a“bogie beam” in some illustrative examples. Beam 1034 is connected towheels 1036 for landing gear assembly 1002.

In one illustrative example, beam 1034 may be configured to pivot aboutpivot point 1035. For example, second end 1032 of second piston 1010 maybe connected to beam 1034 such that movement of second piston 1010 in adirection parallel to axis 1024 causes rotation of beam 1034 about pivotpoint 1035. Rotation of beam 1034 about pivot point 1035 may change theposition of wheels 1036 relative to each other. Similarly, rotation ofbeam 1034 about pivot point 1035 may cause second piston 1010 to move ina direction parallel to axis 1024.

As depicted, third piston 1012 is located between an inner surface ofouter cylindrical structure 1014 of housing 1006 and first piston 1008.Further, third piston 1012 may move in a direction parallel to axis1024.

In these illustrative examples, movement of first piston 1008, secondpiston 1010, and third piston 1012 is controlled by first fluid 1038 andsecond fluid 1040 in actuator 1001. Outer chamber 1048 of actuator 1001is configured to receive first fluid 1038. Inner chamber 1050 ofactuator 1001 is configured to receive second fluid 1040.

Outer chamber 1048 is formed by the space between outer cylindricalstructure 1014 and inner cylindrical structure 1016. In particular, thisspace includes the space surrounded by at least one of the inner surfaceof outer cylindrical structure 1014, the outer surface of innercylindrical structure 1016, and first piston 1008.

In these illustrative examples, the volume of outer chamber 1048 that isconfigured to hold first fluid 1038 is determined by the position offirst piston 1008. For example, the volume of outer chamber 1048 changeswhen first piston 1008 moves in a direction parallel to axis 1024.

Further, third piston 1012 is configured to move in a direction parallelto axis 1024 to cause outer chamber 1048 to divide into firstsub-chamber 1047 and second sub-chamber 1049. The volumes of firstsub-chamber 1047 and second sub-chamber 1049 are determined by theposition of third piston 1012 within outer cylindrical structure 1014.

When third piston 1012 is located at first end 1020 of housing 1006within outer cylindrical structure 1014, the volume of secondsub-chamber 1049 may be substantially zero. However, as third piston1012 moves away from first end 1020 and towards second end 1022 ofhousing 1006, the volume of second sub-chamber 1049 increases and thevolume of first sub-chamber 1047 decreases.

Inner chamber 1050 is formed by inner cylindrical structure 1016 ofhousing 1006, first piston 1008, and second piston 1010. In theseillustrative examples, the volume of inner chamber 1050 configured tohold second fluid 1040 is determined by the position of first piston1008 and the position of second piston 1010. For example, the volume ofinner chamber 1050 is changed when first piston 1008 and/or secondpiston 1010 moves in a direction parallel to axis 1024.

As depicted in these examples, first fluid 1038 comprises hydraulicliquid 1042, and second fluid 1040 comprises gas 1044 and hydraulicliquid 1046. Gas 1044 is a compressible gas in these examples. Forexample, gas 1044 may comprise nitrogen. Of course, in otherillustrative examples, gas 1044 may comprise air, helium, and/or someother suitable type of compressible gas.

Hydraulic liquid 1042 and hydraulic liquid 1046 may be the same type ofhydraulic liquid in these illustrative examples. These hydraulic liquidsmay comprise water, oil, phosphate-ester fluid, and/or other suitabletypes of hydraulic liquids.

Hydraulic liquid 1046 in inner chamber 1050 may be used to lubricate anydevices associated with movement between inner cylindrical structure1016, first piston 1008, and/or second piston 1010 that are exposed ininner chamber 1050. These devices may include, for example, withoutlimitation, any number of bearings, seals, and/or other suitable typesof mechanical devices.

The flow of first fluid 1038 into and out of outer chamber 1048 iscontrolled by manifold 1005 in hydraulic strut assembly 1000, in theseillustrative examples. Manifold 1005 is associated with actuator 1001.Manifold 1005 is a structure comprising channels through which firstfluid 1038 may flow. Any number of valves, ports, sensors, and/or othersuitable components may be associated with this structure in manifold1005 to control the flow of first fluid 1038 through manifold 1005, aswell as the flow of first fluid 1038 into and out of outer chamber 1048.

The amount and pressure of first fluid 1038 in first sub-chamber 1047and second sub-chamber 1049 in outer chamber 1048 may determine theposition of third piston 1012 in outer chamber 1048. For example, asfirst fluid 1038 enters second sub-chamber 1049 and exits firstsub-chamber 1047, third piston 1012 may float upwards through outerchamber 1048 in a direction parallel to axis 1024.

Movement of third piston 1012 may cause movement of first piston 1008.For example, the amount and/or pressure of first fluid 1038 in secondsub-chamber 1049 may be increased such that third piston 1012 movesupwards, towards second end 1022 of housing 1006, in a directionparallel to axis 1024 and pushes against first end 1026 of first piston1008. Third piston 1012 pushes against first end 1026 of first piston1008 in a manner that moves first piston 1008 upwards in a directionparallel to axis 1024. In other words, when third piston 1012 pushesagainst first end 1026 of first piston 1008, first piston 1008 retractsuntil first end 1026 reaches second end 1022 of housing 1006.

Further, in these illustrative examples, first piston 1008 may be fullyextended when the amount and/or pressure of first fluid 1038 in secondsub-chamber 1049 is not sufficient enough to cause third piston 1012 topush against first end 1026 of first piston 1008. In other words,without third piston 1012 pushing against first end 1026 towards secondend 1022 of housing 1006, first piston 1008 may fully extend.

Second fluid 1040 may be introduced into inner chamber 1050 in a numberof different ways. As one illustrative example, an operator may pourhydraulic liquid 1046 into inner chamber 1050 through an open port. Theoperator may subsequently pump gas 1044 into inner chamber 1050. Theoperator may be, for example, a human operator, a robotic operator, orsome other suitable type of operator.

When first piston 1008 and third piston 1012 are in retracted positionsand a load is not being applied to second end 1032 of second piston 1010by beam 1034, the pressure of gas 1044 causes gas 1044 within innerchamber 1050 to push against first end 1030 of second piston 1010 in adirection away from second end 1022 of housing 1006. In other words, thepressure of gas 1044 causes second piston 1010 to extend.

When the amount and/or pressure of first fluid 1038 in secondsub-chamber 1049 is not sufficient enough to cause third piston 1012 andfirst piston 1008 to retract, the pressure of gas 1044 may cause secondpiston 1010 to fully extend. When second piston 1010 is fully extended,first end 1030 of second piston 1010 pushes against second end 1028 offirst piston 1008 in the direction away from second end 1022 of housing1006. Further, when second piston 1010 is fully extended, the volume ofinner chamber 1050 is increased as compared to when second piston 1010is retracted. The extension of second piston 1010 causes gas 1044 toexpand and fill the increased volume of inner chamber 1050.

In these illustrative examples, when a compressive load is applied tosecond end 1032 of second piston 1010 by beam 1034, second piston 1010may retract. This retraction may occur even when first piston 1008 andthird piston 1012 are in retracted positions. When second piston 1010retracts, gas 1044 is compressed. Further, when second piston 1010 isfully retracted, the volume of inner chamber 1050 is decreased ascompared to when second piston 1010 is extended.

When both first piston 1008 and second piston 1010 are fully extended,actuator 1001 is configured to be fully extended. When first piston 1008is fully retracted and second piston 1010 is fully retracted, actuator1001 is considered to be fully compressed. When first piston 1008 isfully retracted and second piston 1010 is fully extended, actuator 1001is configured to be retracted.

As depicted, second piston 1010 may have an open end to increase thevolume of inner chamber 1050. Second piston 1010 may also have elongatemember 1052. Elongate member 1052 may be configured to divide innerchamber 1050 into first sub-chamber 1051 and second sub-chamber 1053.First sub-chamber 1051 is located within inner cylindrical structure1016. Second sub-chamber 1053 is located within second piston 1010.

In particular, elongate member 1052 may have open ends such that gas1044 in inner chamber 1050 may move between first sub-chamber 1051 andsecond sub-chamber 1053. In one illustrative example, elongate member1052 may extend into first sub-chamber 1051 beyond a fluid line forhydraulic liquid 1046 in inner chamber 1050. In this manner, the chancesof hydraulic liquid 1046 entering the cavity inside second piston 1010may be reduced. In another illustrative example, elongate member 1052may extend into second sub-chamber 1053 to draw any hydraulic liquid1046 from second sub-chamber 1053 into first sub-chamber 1051. Thisaction may occur when at least one of the second piston 1010 and thefirst piston 1008 extends.

In these illustrative examples, gas 1044 in inner chamber 1050 allowssecond piston 1010 to retract without the resistance of hydraulic liquidmotion in response to wheels 1036 contacting ground with actuator 1001in a fully extended state. Control of the positions and movement offirst piston 1008, second piston 1010, and third piston 1012 using firstfluid 1038 and second fluid 1040 is described in greater detail withrespect to a particular implementation for hydraulic strut assembly 1100in FIG. 11 below.

The illustration of hydraulic strut assembly 1000 in FIG. 10 is notmeant to imply physical or architectural limitations to the manner inwhich an illustrative embodiment may be implemented. Other components inaddition to or in place of the ones illustrated may be used. Somecomponents may be unnecessary. Also, the blocks are presented toillustrate some functional components. One or more of these blocks maybe combined, divided, or combined and divided into different blocks whenimplemented in an illustrative embodiment

For example, in some illustrative examples, pistons in addition to firstpiston 1008, second piston 1010, and third piston 1012 may be present inactuator 1001. In other illustrative examples, manifold 1005 may includecomponents not described above. For example, manifold 1005 may includevalves not described in FIG. 10.

FIG. 11 is an illustration of a cross-sectional view of a hydraulicstrut assembly, depicted in accordance with an illustrative embodiment.In this illustrative example, hydraulic strut assembly 1100 is anexample of one implementation for hydraulic strut assembly 1000 in FIG.10. Hydraulic strut assembly 1100 may be used in a landing gearassembly, such as, for example, landing gear assembly 1002 in FIG. 10.

As depicted, hydraulic strut assembly 1100 comprises actuator 1102 andmanifold 1104. Actuator 1102 is an example of one implementation foractuator 1001 in FIG. 10. Manifold 1104 is an example of oneimplementation for manifold 1005 in FIG. 10.

In this illustrative example, actuator 1102 includes housing 1106, firstpiston 1116, second piston 1118, and third piston 1120. Housing 1106,first piston 1116, second piston 1118, and third piston 1120 areexamples of one implementation for housing 1006, first piston 1008,second piston 1010, and third piston 1012, respectively, in FIG. 10.

In this illustrative example, housing 1106 comprises outer cylindricalstructure 1108 and inner cylindrical structure 1112. Further, housing1106 has first end 1113 and second end 1115.

First piston 1116, second piston 1118, and third piston 1120 areassociated with housing 1106 in this depicted example. First piston 1116has first end 1117 and second end 1119. Second piston 1118 has first end1121 and second end 1123.

First piston 1116, second piston 1118, and third piston 1120 areconfigured to move linearly in a direction parallel to axis 1125. Axis1125 is a center axis through actuator 1102 in this depicted example. Inparticular, first piston 1116 may move in a direction parallel to axis1125 relative to first end 1113 of housing 1106. Movement of firstpiston 1116 away from second end 1115 of housing 1106 is extension.Movement of first piston 1116 toward second end 1115 of housing 1106 isretraction.

Second piston 1118 may move in a direction parallel to axis 1125relative to second end 1119 of first piston 1116. Movement of secondpiston 1118 toward second end 1115 of housing 1106 is retraction.Movement of second piston 1118 away from second end 1115 of housing 1106is extension. When second piston 1118 retracts, second end 1123 ofsecond piston 1118 may contact spring system 1127.

Spring system 1127 may comprise one or more springs associated withsecond end 1119 of first piston 1116 and/or second end 1123 of secondpiston 1118. Spring system 1127 may comprise, for example, at least oneof a mechanical spring, a coil spring, a ring spring, a leaf spring, anelastomeric spring, and some other suitable of spring device.

Spring system 1127 is configured to compress in response to a loadapplied to spring system 1127 by second end 1123 of second piston 1118and/or second end 1119 of first piston 1116. Spring system 1127 reducesthe acceleration and/or force with which first piston 1116 retracts whensecond end 1123 of second piston 1118 contacts spring system 1127.

Further, spring system 1127 prevents second end 1123 of second piston1118 from directly contacting second end 1119 of first piston 1116 whensecond piston 1118 retracts. In this manner, undesired effects to secondend 1119 of first piston 1116 that may be caused by second end 1123 ofsecond piston 1118 contacting second end 1119 of first piston 1116 maybe prevented.

In this illustrative example, third piston 1120 is located between firstpiston 1116 and the inner surface of outer cylindrical structure 1108 ofhousing 1106. As depicted, third piston 1120 may move between firstpiston 1116 and outer cylindrical structure 1108 in a direction parallelto axis 1125. When third piston 1120 moves upwards towards second end1115 of housing 1106, third piston 1120 may push first end 1117 of firstpiston 1116 towards second end 1115 of housing 1106, causing firstpiston 1116 to retract. When third piston 1120 moves away from secondend 1115 of housing 1106, first piston 1116 is allowed to extend.

As depicted, outer chamber 1122 is formed in the space surrounded byouter cylindrical structure 1108, inner cylindrical structure 1112, andfirst piston 1116. Third piston 1120 divides outer chamber 1122 intofirst sub-chamber 1141 and second sub-chamber 1143. Movement of thirdpiston 1120 in the direction parallel to axis 1125 causes the volumes offirst sub-chamber 1141 and second sub-chamber 1143 to change.

Further, inner chamber 1124 is formed by inner cylindrical structure1112, first piston 1116 and second piston 1118. Movement of secondpiston 1118 in a direction parallel to axis 1125 changes a volume ofinner chamber 1124. In particular, the volume of inner chamber 1124increases when second piston 1118 extends, and the volume of innerchamber 1124 decreases when second piston 1118 retracts. Additionally,extension of first piston 1116 also may increase the volume of innerchamber 1124.

In this illustrative example, second piston 1118 has tube 1128. Tube1128 is an example of one implementation for elongate member 1052 inFIG. 10. Both ends of tube 1128 are open in this example. In thismanner, tube 1128 connects first sub-chamber 1147 of inner chamber 1124to second sub-chamber 1149 of inner chamber 1124. Second sub-chamber1149 of inner chamber 1124 is formed by cavity 1126 inside second piston1118. In some illustrative examples, one or both ends of tube 1128 maybe partially open or partially covered.

Charge valve 1130 and pressure sensor 1132 are associated with secondend 1115 of housing 1106 in this depicted example. Charge valve 1130provides a mechanism for adding fluid to inner chamber 1124. Inparticular, both a hydraulic liquid and a compressible gas may be addedto inner chamber 1124 through charge valve 1130. Alternatively, aseparate port may be used to fill and/or drain hydraulic liquid frominner chamber 1124. Also, in some illustrative examples, port 1139located near second end 1123 of second piston 1118 may be used forfilling and/or draining fluid from inner chamber 1124. Pressure sensor1132 is configured to measure the pressure of a compressible gas held ininner chamber 1124.

In this illustrative example, manifold 1104 of hydraulic strut assembly1100 is associated with actuator 1102. Manifold 1104 is depicted in theform of a schematic in this depicted example. As depicted, manifold 1104has plurality of channels 1131 and plurality of valves 1133 throughwhich a hydraulic liquid may flow within manifold 1104.

The hydraulic liquid flowing through manifold 1104 may enter secondsub-chamber 1143 of outer chamber 1122 through channel 1135 in pluralityof channels 1131. The hydraulic liquid in first sub-chamber 1141 ofouter chamber 1122 may return to manifold 1104 through channel 1137 inplurality of channels 1131.

Hydraulic liquid enters manifold 1104 from source 1134. Source 1134 maybe any suitable type of supply of hydraulic liquid. For example, source1134 may be a container or tank filled with hydraulic liquid. Thehydraulic fluid in source 1134 may have sufficient pressure to enablemovement of fluid through manifold 1104 and into second sub-chamber 1143of outer chamber 1122 to facilitate movement of third piston 1120 andfirst piston 1116.

Hydraulic liquid flows from source 1134 into manifold 1104 throughfilter 1136. Further, hydraulic liquid may flow from manifold 1104 intoreturn 1145. Return 1145 may take the form of, for example, withoutlimitation, a storage container, a tank, or some other suitablecomponent configured to hold hydraulic liquid received from manifold1104.

The flow of hydraulic liquid through manifold 1104 is controlled usingplurality of valves 1133. Plurality of valves 1133 includes valve 1138,valve 1140, valve 1142, and valve 1144. Valve 1138 may be a multi-modepressure-reducing valve in this depicted example. Further, valve 1140may be a first solenoid shut-off valve, and valve 1142 may be a secondsolenoid shut-off valve. Valve 1144 may be a multi-mode pressure-reliefvalve.

Spring 1146 indicates an “at rest” mode of operation for valve 1138. Asdepicted, when valve 1138 is at rest, hydraulic liquid flowing fromfilter 1136 is allowed to flow through channel 1135 and into secondsub-chamber 1143 of outer chamber 1122. Valve 1138 is at rest when thepressure of hydraulic liquid at input 1150 has not reached a selectedlevel and the pressure of hydraulic liquid at input 1153 has not reacheda selected level.

When the actuator is to be placed in a retracted position, valve 1138reduces pressure of source 1134 by sensing the outlet pressure atchannel 1135 and compressing spring 1146 to adjust valve position andmaintain a selected pressure level. Valve 1138 senses the pressure ofchannel 1135 as input 1153 into valve 1138. In this manner, hydraulicliquid may have a pressure of about 800 psi and may flow into secondsub-chamber 1143 of outer chamber 1122 through channel 1135, retractingthird piston 1120 and first piston 1118.

Valve 1138 may change positions based on input 1150, input 1152, andinput 1153. When actuator 1102 is to be fully extended, the pressure ofhydraulic liquid at input 1150 is increased to a selected level orgreater. Consequently, the pressure level at input 1150 pushes againstspring 1146 and valve 1138 changes position to allow the flow ofhydraulic liquid from second sub-chamber 1143 of outer chamber 1122 intoplurality of channels 1131 and valve 1138. This hydraulic fluid may exitmanifold 1104 at return 1145. In this illustrative example, the outletpressure for valve 1138 in channel 1135 may be about 70 psi.

The pressure of hydraulic liquid at the outlet of valve 1138 and channel1135 is reduced such that third piston 1120 moves downwards freely. Thismovement of third piston 1120 allows first piston 1116 and second piston1118 to extend such that actuator 1102 fully extends.

When the actuator is to be placed in a retracted position and locked,the pressure of hydraulic liquid at input 1152 is increased to aselected level or greater. Consequently, valve 1138 is blocked frommoving to allow hydraulic liquid having a pressure substantially equalto source 1134 to flow through valve 1138 and into second sub-chamber1143 of outer chamber 1122 through channel 1135. Pressure of sourcefluid 1134 may be, for example, without limitation, about 5000 psi.

Further, spring 1154 for valve 1140 indicates an at rest mode ofoperation for valve 1140. When valve 1140 is at rest, hydraulic liquidflowing from filter 1136 is allowed to pass through valve 1140 to input1150 for valve 1138 and to input 1162 for valve 1144. Valve 1140 is atrest when solenoid actuator 1156 associated with valve 1140 is notactivated. Solenoid actuator 1156 may be activated in response toelectrical signals.

These electrical signals may be received from a control system locatedonboard the aircraft.

When solenoid actuator 1156 is activated, spring 1154 is compressed andvalve 1140 changes position to allow hydraulic liquid to flow towardsreturn 1145 to reduce pressure levels at input 1162 for valve 1144 andinput 1150 for valve 1138. Solenoid actuator 1156 may be activated whenactuator 1102 is to be moved to the reacted position in which firstpiston 1116 is retracted and second piston 1118 is extended.

Spring 1158 for valve 1142 indicates an at rest mode of operation forvalve 1142. When valve 1142 is at rest, hydraulic liquid is blocked fromflowing through valve 1142. When actuator 1102 is to be retracted andlocked, solenoid actuator 1160 associated with valve 1142 is activated.This activation causes spring 1158 to compress and valve 1142 to changeposition such that hydraulic liquid flowing from filter 1136 may flowthrough valve 1142 towards input 1152 for valve 1138 and input 1155 forvalve 1144. This flow of hydraulic liquid towards input 1152 for valve1138 increases the pressure at input 1152 and blocks valve 1138 frommoving out of the at rest state. Further, the flow of hydraulic liquidtowards input 1155 for valve 1144 increases the pressure at input 1155and changes the pressure level at which valve 1144 will open and allowhydraulic liquid to flow through valve 1144.

Valve 1144 is configured to open to allow hydraulic liquid to flow fromchannel 1129 into channel 1163 when the pressure of the hydraulic liquidat channel 1129 reaches either a low pressure setting or a high pressuresetting selected for valve 1144. The low pressure setting for valve 1144may be any pressure value that is between the pressure value for source1134 and the pressure value for return 1145. The high pressure settingfor valve 1144 may be higher than the pressure value for source 1134.

The pressure value for source 1134 may be referred to as the sourcepressure. The pressure value for return 1145 may be referred to as thereturn pressure. If the source pressure is about 5000 psi, and thereturn pressure is about 70 psi, then the low pressure setting for valve1144 may be about 1000 psi and the high pressure setting for valve 1144may be about 5500 psi.

When the pressure of the hydraulic liquid at channel 1129 decreases to apressure value that is lower than the low pressure setting for valve1144, valve 1144 closes and blocks the flow of hydraulic liquid throughvalve 1144 into channel 1163. Additionally, when the pressure ofhydraulic liquid at input 1162 reaches the desired level of pressure,valve 1144 will open to allow hydraulic liquid to flow from channel 1129into channel 1163.

In this manner, plurality of valves 1133 controls the flow of hydraulicliquid through manifold 1104 as well as into and out of outer chamber1122. Further, pressure sensor 1164 is configured to measure thepressure of hydraulic liquid within outer chamber 1122.

Additionally, seal system 1165 may be associated with first end 1121 ofsecond piston 1118. Seal system 1165 may comprise any number of sealsconfigured to allow second piston 1118 to move relative to innercylindrical structure 1112 and provide a seal for inner chamber 1124. Inparticular, this seal for inner chamber 1124 is formed when secondpiston 1118 is retracted into inner cylindrical structure 1112 and sealsystem 1165 engages with the inner surface of inner cylindricalstructure 1112.

Seal system 1165 may be configured to divide first sub-chamber 1147 ofinner chamber 1124 into a first portion and a second portion. The firstportion may be the portion of first sub-chamber 1147 above seal system1165 and the second portion may be the portion of first sub-chamber 1147below seal system 1165. As second piston 1118 is retracted into innercylindrical structure 1112, the volume of the first portion of firstsub-chamber 1147 decreases and the volume of the second portion of firstsub-chamber 1147 increases. Alternately, as second piston 1118 isextended, the volume of the first portion of first sub-chamber 1147increases and the volume of the second portion of first sub-chamber 1147decreases. The change in the volume of the first portion and the secondportion of first sub-chamber 1147 causes the hydraulic liquid portion ofthe second fluid to be forced past seal system 1165 to provideresistance to the movement of the second piston 1118.

Further, retraction of second piston 1118 causes the seal formed by sealsystem 1165 to be moved.

FIGS. 12 through 14 are illustrations of different positions for thepistons in an actuator, depicted in accordance with an illustrativeembodiment. In FIGS. 12 through 14, actuator 1102 from FIG. 11 isdepicted in a compressed position, a retracted position, and an extendedposition, respectively.

FIG. 12 is an illustration of actuator 1102 in a compressed position,depicted in accordance with an illustrative embodiment. In thisillustrative example, actuator 1102 has compressed position 1200.Actuator 1102 may have compressed position 1200 when, for example, theaircraft in which actuator 1102 is used is parked on the ground.

As depicted, third piston 1120 and first piston 1116 are in retractedpositions 1202 and 1204, respectively. In other words, third piston 1120has moved upwards such that first piston 1116 is fully retracted suchthat first end 1117 of first piston 1116 is at second end 1115 ofhousing 1106.

In this illustrative example, a compressive load has been applied tosecond end 1123 of second piston 1118 to cause second piston 1118 topartially retract. In particular, first end 1121 of second piston 1118has moved upwards and away from second end 1119 of first piston 1116.The load applied to second end 1123 of second piston 1118 may be a loadtransferred to second end 1123 of second piston 1118 in response to thelanding gear assembly contacting the ground.

The positions of first piston 1116, second piston 1118, and third piston1120 in FIG. 12 may be determined by first fluid 1206 in outer chamber1122 and second fluid 1208 in inner chamber 1124. First fluid 1206 is ahydraulic liquid. This hydraulic liquid may be introduced into outerchamber 1122 by manifold 1104 in FIG. 11. Further, first fluid 1206 mayflow out of outer chamber 1122 and return to manifold 1104 in FIG. 11.

Second fluid 1208 comprises both gas 1210 and hydraulic liquid 1212. Gas1210 is a compressible gas. When second piston 1118 retracts, gas 1210compresses. The compressibility of gas 1210 allows for rapid compressionof the second piston without the resistance of fluid flow. Second fluid1208 may be introduced into inner chamber 1124 through charge valve 1130in FIG. 11. Hydraulic liquid 1212 is used to lubricate seal system 1165and reduce undesired effects to seal system 1165 in response tooperation of actuator 1102.

When first piston 1116 is fully retracted, seal system 1165 forms a sealbetween first end 1121 of second piston 1118 and a lower end of innercylindrical structure 1112. When second piston 1118 moves, second fluid1208 will flow past seal system 1165. Seal system 1165 may include asmall fluid passage to allow the speed of movement of second piston 1118to be reduced. In this manner, bogie beam motion may be dampened whenthe aircraft is traveling on the ground.

As depicted, tube 1128 extends into inner chamber 1124 above a fluidline for hydraulic liquid 1212. In this manner, the possibility ofhydraulic liquid 1212 entering cavity 1126 may be reduced. However, gas1210 may be allowed to expand into cavity 1126.

As depicted in FIG. 12, tube 1128 extends into inner chamber 1124 suchthat hydraulic liquid 1212 does not enter cavity 1126. In some cases, aportion of hydraulic liquid 1212 may enter cavity 1126 through tube 1128when actuator 1102 is compressed. In these examples, extension of secondpiston 1118 and/or first piston 1116 may cause gas 1210 to expand suchthat a pressure within inner chamber 1124 is reduced. The pressure ininner chamber 1124 may be reduced to a level below the pressure incavity 1126. Consequently, any gas 1210 and/or any hydraulic liquid 1212in cavity 1126 may be expelled from cavity 1126, having a higherpressure, into inner chamber 1124, having a lower pressure, through tube1128.

FIG. 13 is an illustration of actuator 1102 in a retracted position,depicted in accordance with an illustrative embodiment. In thisillustrative example, actuator 1102 has retracted position 1300. Inparticular, third piston 1120 has retracted position 1202, first piston1116 has retracted position 1204, and second piston 1118 is fullyextended when actuator 1102 has retracted position 1300.

Actuator 1102 may be configured to have retracted position 1300 whenactuator 1102 is to be in a landing position and/or a locked position.Actuator 1102 may be in a landing position when, for example, theaircraft has prepared the landing gear for landing onto the ground.Actuator 1102 may be in a locked position when, for example, theaircraft is traveling on a runway in preparation for takeoff.

FIG. 14 is an illustration of actuator 1102 in a fully extended positiondepicted in accordance with an illustrative embodiment. In thisillustrative example, actuator 1102 is considered to be in fullyextended position 1400. In particular, both first piston 1116 and secondpiston 1118 are fully extended when actuator 1102 is in fully extendedposition 1400.

Actuator 1102 may be in fully extended position 1400 when the amountand/or pressure of first fluid 1206 from FIGS. 12-13 in secondsub-chamber 1143 of outer chamber 1122 has been reduced such thatpressure of second fluid 1208 forces gas 1210 to expand, first piston1116 and second piston 1118 to extend, and third piston 1120 to movedownwards.

FIG. 15 is an illustration of a landing gear assembly with an actuatorin a compressed position depicted in accordance with an illustrativeembodiment. In this illustrative example, landing gear assembly 600 fromFIG. 6 is depicted having hydraulic strut assembly 1100 from FIG. 11instead of hydraulic strut 606 in FIG. 6. Manifold 1104 for hydraulicstrut assembly 1100 may not be seen in this view. As depicted, landinggear assembly 600 is in a ground position. In this ground position,actuator 1102 has compressed position 1200 from FIG. 12.

FIG. 16 is an illustration of a landing gear assembly with an actuatorin a retracted position depicted in accordance with an illustrativeembodiment. In this illustrative example, landing gear assembly 600 fromFIG. 8 is depicted having hydraulic strut assembly 1100 from FIG. 11instead of hydraulic strut 606 in FIG. 8. As depicted, landing gearassembly 600 is in a landing position. In this landing position,actuator 1102 has retracted position 1300 from FIG. 13.

FIG. 17 is an illustration of a landing gear assembly with an actuatorin a fully extended position depicted in accordance with an illustrativeembodiment. In this illustrative example, landing gear assembly 600 fromFIG. 7 is depicted having hydraulic strut assembly 1100 from FIG. 11instead of hydraulic strut 606 in FIG. 7. As depicted, landing gearassembly 600 is in a stow position. In this stow position, actuator 1102has fully extended position 1400 from FIG. 14.

FIG. 18 is an illustration of a flowchart of a method of operating ahydraulic actuator in an aircraft, in accordance with an illustrativeembodiment. The process shown in FIG. 18 may be implemented using ahydraulic actuator assembly 200, such as that shown in FIG. 2 throughFIG. 5, or may be implemented using a hydraulic strut 606, such as thatshown in FIGS. 6 through 8.

The process 1800 begins by operating a vehicle, the vehicle comprising:a fuselage; a wing connected to the fuselage; a landing gear assemblyconnected to one of the fuselage and the wing; an actuator connected tothe landing gear assembly, wherein the actuator comprises: a firsthydraulic piston; a second hydraulic piston disposed within the firsthydraulic piston; and a third hydraulic piston disposed within both thefirst hydraulic piston and the second hydraulic piston, wherein thefirst, second, and third hydraulic pistons are contained within a commonouter wall; and a manifold is contained within the common outer wall,the manifold disposed relative to the first, second, and third hydraulicpistons such that a fluid moving in the manifold can control positionsof the first, second, and third hydraulic pistons (operation 1802). Inan illustrative embodiment the method may include, during liftoff,passively pulling the second hydraulic piston (operation 1804). In anillustrative embodiment, the method may further include, while stowingthe landing gear assembly, extending the first, second, and thirdhydraulic pistons (operation 1806).

In an illustrative embodiment, the method may further include, whilepositioning for landing, retracting the first hydraulic piston so that abogie beam connected to the landing gear assembly is positioned suchthat a forward axle of the bogie beam is disposed upwardly relative to arear axle of the bogie beam (operation 1808). In an illustrativeembodiment, the method may further include reacting to an overloadcondition by compressing the first, second, and third hydraulic pistons(operation 1810). In an illustrative embodiment, the method may furtherinclude forcing fluid with respect to the second hydraulic piston suchthat the actuator acts as a dampener (operation 1812). The processterminates thereafter.

Thus, the illustrative embodiments provide for an actuator. The actuatorincludes a first hydraulic piston, a second hydraulic piston disposedwithin the first hydraulic piston, and a third hydraulic piston disposedwithin both the first hydraulic piston and the second hydraulic piston.The first, second, and third hydraulic pistons are contained within acommon outer wall.

The illustrative embodiments present provide for a nested pistonactuator that is flexible, durable, light weight, and relativelyinexpensive compared to other actuators. Additionally, the illustrativeembodiments have added further value to aircraft operation in that theillustrative embodiments aid an aircraft in both landing and lift off.The illustrative embodiments aid an aircraft to lift off by increasingthe height of the landing gear assembly at the time of initial take-offrotation, which allows a higher angle of attack. Other illustrativeembodiments are apparent from the following additional description.

FIG. 19 is an illustration of a process for operating a vehicle duringan alternate landing in the form of a flowchart depicted in accordancewith an illustrative embodiment. The process described in FIG. 19 may beimplemented using hydraulic strut assembly 1000 in FIG. 10. For example,this process may be used to operate aircraft 1004 in FIG. 10 whenaircraft 1004 performs an alternate landing.

The process begins by operating the aircraft during an alternate landingin which the aircraft comprises a landing gear assembly with an actuatorcomprising a housing, a first piston, and a second piston (operation1900). During an alternate landing, the landing gear assembly for theaircraft may be in a stow position. In particular, the actuator in thelanding gear assembly may be in a fully extended position.

In this illustrative example, the housing of the actuator comprises anouter cylindrical structure and an inner cylindrical structure. Theouter cylindrical structure and the inner cylindrical structure form anouter chamber configured to receive a first fluid. The first piston isconfigured to move in a direction parallel to an axis through thehousing relative to a first end of the housing.

Further, the second piston is configured to move in the directionparallel to the axis through the housing relative to a second end of thefirst piston such that a volume of an inner chamber formed by the innercylindrical structure and the second piston changes. The inner chamberis configured to hold a second fluid in which the second fluid comprisesa gas that is compressible.

Thereafter, the process retracts the second piston and then the firstpiston in response to a load being applied to the second piston when thelanding gear assembly contacts a ground on which the aircraft is landing(operation 1902). The gas in the inner chamber compresses when thesecond piston and the first piston retract. In operation 1902, the gasallows the second piston and the first piston to be retracted when theaircraft touches the ground during the alternate landing. Further, withthe compressible gas in the inner chamber, the second fluid that ispresent in the inner chamber is not expelled from the inner chamberduring the retraction of the second piston and the first piston.

The flowcharts and block diagrams in the different depicted illustrativeembodiments illustrate the architecture, functionality, and operation ofsome possible implementations of apparatus and methods in differentillustrative embodiments. In this regard, each block in the flowchart orblock diagrams may represent a module, segment, function, and/or aportion of an operation or step. The illustrative embodiments may bemanufactured or configured to perform one or more operations in theflowcharts or block diagrams.

In some alternative implementations, the function or functions noted inthe block may occur out of the order noted in the figures. For example,in some cases, two blocks shown in succession may be executedsubstantially concurrently, or the blocks may sometimes be executed inthe reverse order, depending upon the functionality involved. Also,other blocks may be added in addition to the illustrated blocks in aflowchart or block diagram.

Illustrative embodiments of the disclosure may be described in thecontext of aircraft manufacturing and service method 2000 as shown inFIG. 20 and aircraft 2100 as shown in FIG. 21. Turning first to FIG. 20,an illustration of an aircraft manufacturing and service method isdepicted in accordance with an illustrative embodiment. Duringpre-production, aircraft manufacturing and service method 2000 mayinclude specification and design 2002 of aircraft 2100 in FIG. 21 andmaterial procurement 2004.

During production, component and subassembly manufacturing 2006 andsystem integration 2008 of aircraft 2100 in FIG. 21 takes place.Thereafter, aircraft 2100 in FIG. 21 may go through certification anddelivery 2010 in order to be placed in service 2012. While in service2012 by an operator, aircraft 2100 in FIG. 21 is scheduled for routinemaintenance and service 2014, which may include modification,reconfiguration, refurbishment, and other maintenance or service.

Each of the processes of aircraft manufacturing and service method 2000may be performed or carried out by a system integrator, a third party,and/or an operator. For the purposes of this description, a systemintegrator may include, without limitation, any number of aircraftmanufacturers and major-system subcontractors; a third party mayinclude, without limitation, any number of vendors, subcontractors, andsuppliers; and an operator may be an airline, a leasing company, amilitary entity, a service organization, and so on.

With reference now to FIG. 21, an illustration of an aircraft isdepicted in which an illustrative embodiment may be implemented. In thisexample, aircraft 2100 is produced by aircraft manufacturing and servicemethod 2000 in FIG. 20 and may include airframe 2102 with plurality ofsystems 2104 and interior 2106. Examples of systems 2104 include one ormore of propulsion system 2108, electrical system 2110, hydraulic system2112, environmental system 2114, and landing gear system 2116. Landinggear system 2116 may include one or more landing gear assemblies suchas, for example, without limitation, landing gear assembly 108 in FIG.1, landing gear assembly 600 in FIGS. 6 through 8, landing gear 902 inFIG. 9, landing gear assembly 1002 in FIG. 10, or landing gear assembly600 in FIGS. 16 through 17. Any number of other systems may be includedin systems 2104, depending on the implementation.

Apparatus and methods embodied herein may be employed during at leastone of the stages of aircraft manufacturing and service method 2000 inFIG. 20. For example, actuator 110 in FIG. 1, hydraulic actuatorassembly 200 in FIGS. 2 through 5, hydraulic strut 606 in FIGS. 6through 8, actuator 910 in FIG. 9, or hydraulic strut assembly 1000 maybe formed and added to landing gear system 2116 for aircraft 2100 duringat least one of component and subassembly manufacturing 2206, systemintegration 2208, and maintenance and service 2014.

In one illustrative example, components or subassemblies produced incomponent and subassembly manufacturing 2006 in FIG. 20 may befabricated or manufactured in a manner similar to components orsubassemblies produced while aircraft 2100 is in service 2012 in FIG.20. As yet another example, one or more apparatus embodiments, methodembodiments, or a combination thereof may be utilized during productionstages, such as component and subassembly manufacturing 2006 and systemintegration 2008 in FIG. 20. One or more apparatus embodiments, methodembodiments, or a combination thereof may be utilized while aircraft2100 is in service 2012 and/or during maintenance and service 2014 inFIG. 20. The use of a number of the different illustrative embodimentsmay substantially expedite the assembly of and/or reduce the cost ofaircraft 2100.

As used herein, the phrase “at least one of”, when used with a list ofitems, means that different combinations of one or more of the listeditems may be used and only one of each item in the list may be needed.For example, “at least one of item A, item B, and item C” may include,for example, without limitation, item A or item A and item B. Thisexample also may include item A, item B, and item C, or item B and itemC. In other examples, “at least one of” may be, for example, withoutlimitation, two of item A, one of item B, and ten of item C; four ofitem B and seven of item C; and other suitable combinations.

Thus, the illustrative embodiments provide for a guide tube and afloating piston disposed within the guide tube. The floating piston isconfigured within the guide tube such that a landing gear connected tothe floating piston may extend rapidly to the stow position relative toa mechanical device for retracting a landing gear.

The illustrative embodiments also provide for an actuator including afirst hydraulic piston, a second hydraulic piston disposed within thefirst hydraulic piston, and a third hydraulic piston disposed withinboth the first hydraulic piston and the second hydraulic piston. Thefirst, second, and third hydraulic pistons are contained within a commonouter wall. A manifold is connected to the first, second, and thirdhydraulic pistons. The manifold is disposed relative to the first,second, and third hydraulic pistons such that a fluid moving in themanifold can control positions of the first, second, and third hydraulicpistons.

The embodiments also provide for a vehicle including a fuselage, a wingconnected to the fuselage, and a landing gear assembly connected to atleast one of the fuselage and the wing. The vehicle further includes ahydraulic actuator connected to the landing gear assembly. The hydraulicactuator includes a first hydraulic piston, a second hydraulic pistondisposed within the first hydraulic piston, and a third hydraulic pistondisposed within both the first hydraulic piston and the second hydraulicpiston. The first, second, and third hydraulic pistons are containedwithin a common outer wall. The hydraulic actuator further includes amanifold connected to the first, second, and third hydraulic pistons.The manifold is disposed relative to the first, second, and thirdhydraulic pistons such that a fluid moving in the manifold can controlpositions of the first, second, and third hydraulic pistons.

The embodiments also provide for a method for operating a vehicle. Thevehicle includes a fuselage, a wing connected to the fuselage, and alanding gear assembly connected to one of the fuselage or the wing. Anactuator is connected to the landing gear assembly. The actuatorincludes a first hydraulic piston, a second hydraulic piston disposedwithin the first hydraulic piston, and a third hydraulic piston disposedwithin both the first hydraulic piston and the second hydraulic piston.The first, second, and third hydraulic pistons are contained within acommon outer wall. A manifold is connected to the first, second, andthird hydraulic pistons. The manifold is disposed relative to the first,second, and third hydraulic pistons such that a fluid moving in themanifold can control positions of the first, second, and third hydraulicpistons.

Further, the different illustrative embodiments provide a hydraulicactuator, such as actuator 1001 in FIG. 10, configured to use acompressible gas that may not need to be expelled from the hydraulicactuator when the hydraulic actuator is rapidly compressed during analternate landing. In one illustrative embodiment, a hydraulic strutassembly comprises a housing, a first piston, a second piston, and athird piston. The housing comprises outer and inner cylindricalstructures. An outer chamber is configured to receive a first fluid thatis formed between the outer cylindrical structure and the innercylindrical structure. The first piston is positioned between the outerand inner cylindrical structures. The second piston is nested within thefirst piston. The inner cylindrical structure, the first piston, and thesecond piston form an inner chamber in which a volume of the innerchamber changes when at least one of the first and second pistons move.The inner chamber is configured to hold a second fluid comprising a gas.The third piston is positioned between the outer cylindrical structureand the first piston. The first, second, and third pistons areconfigured to move in a direction parallel to an axis through thehousing.

The telescopic feature of the actuator in the telescopic hydraulic strutassembly provided by the different illustrative embodiments allows for asmaller overall package than the currently available and/or proposedsystems. The valves within the manifold are configured to controlpressure within the actuator, thereby allowing the telescopic hydraulicstrut assembly to serve multiple functions. The more compact design hasreduced weight as compared to currently available systems and may beused on aircraft landing gears that do not have sufficient space forsome of the currently available semi-levered gear systems.

The actuator provided by the different illustrative embodimentscomprises a fixed outer housing assembly with one closed end, the headend, one open end, and the rod end that houses three moveable pistons.The outer cylinder structure of the actuator is pivotally connected tothe upper portion of the landing gear shock strut. One of the threepistons, the second piston, is pivotally connected to the forward end ofthe bogie beam of the landing gear. The three pistons slide axiallywithin the outer housing. Movement of the piston may be controlled bythe movement of fluids in or out of the actuator, as controlled by thevalve module, or by the landing gear as it pushes or pulls against theattachment ends of the actuator. The valve manifold, when connected to apressurized hydraulic fluid delivery system, will control fluid flow inand out of the actuator with the use of electrically commanded solenoidcontrol valves, pressure reducing valves, pressure relief valves, and/orcheck valves.

In another illustrative embodiment, the hydraulic strut assemblycomprises a housing, a first piston, a second piston, and a thirdpiston. The housing comprises an outer cylindrical structure and aninner cylindrical structure. The inner cylindrical structure, the firstpiston, and the second piston form an inner chamber configured to hold afluid comprising a gas and a hydraulic liquid. The inner chamber mayhave a first sub-chamber and a second sub-chamber. The gas iscompressible such that at least one of the first piston and the secondpiston may be retracted without expelling the second fluid from theinner chamber.

Further, the hydraulic strut assembly may also include a seal systemassociated with at least one of the first piston and the second piston.The seal system may be configured to divide the first sub-chamber of theinner chamber into a first portion and a second portion and provide aseal between the first portion and the second portion of the firstsub-chamber when the second piston retracts. When a volume of the firstportion of the first sub-chamber increases and a volume of the secondportion of the second sub-chamber decreases in response to the secondpiston extending, the hydraulic liquid of the second fluid may be movedpast the seal system to provide resistance to the movement of the secondpiston. Additionally, the hydraulic liquid may provide lubrication forthe seal system.

In one illustrative embodiment, the actuator is fully extended toposition the landing gear bogie beam in a configuration with the forwardaxle lower than the aft axle, such as actuator 1102 in fully extendedposition 1400 in FIG. 14 and FIG. 17. This position may facilitatestowage of the landing gear within an aircraft wheel well. The actuatorachieves the fully extended position when the control valves open theouter chambers to the external hydraulic fluid return system. The innerchamber contains a compressible mixture of fluid that is charged to apredetermined pressure during maintenance of the hydraulic strutassembly. The pressure within the inner chamber forces the movement ofthe pistons to extend. Extension rate of the pistons is controlled by avariable restriction of the fluid that is expelled from the outerchamber of the actuator.

In another illustrative embodiment, the actuator is partially retractedto position the landing gear bogie beam in a configuration with theforward axle higher than the aft axle, such as actuator 1102 inretracted position 1300 in FIG. 13 and FIG. 16. This position mayposition the bogie beam in the optimum touchdown configuration. Theactuator achieves the partially retracted position when the manifoldprovides control of hydraulic fluid to and from the outer chambers. Thisaction retracts the first and third pistons. The inner chamber containsa compressible fluid that is charged to a predetermined pressure duringmaintenance of the hydraulic strut assembly. The pressure within theinner chamber forces the movement of the second piston to extend.

The semi-levered function is enabled by forcibly holding the thirdpiston retracted against the outer housing and thereby preventing theextension of the first and third pistons as the aircraft rotates fortakeoff and the landing gear bogie beam tries to rotate the forward axleaway from the aircraft reference. This motion will attempt to extend theactuator pistons. With highly pressurized hydraulic fluid applied by thecontrol valves to the outer chamber, the fluid acts against the area ofthe third piston to resist the pulling force of the bogie beam andprevent extension of the first and third pistons. A pressure reliefvalve prevents over-pressurization of the fluid in the outer chamber.

In one illustrative embodiment, the aircraft is situated with all tireson the ground and the actuator position is controlled by the landinggear. The actuator may be in a compressed position, such as actuator1102 in compressed position 1200 in FIG. 12 and FIG. 15. The actuatorlength may be influenced by the angle of the bogie beam with respect tothe landing gear shock strut and the extension amount of the landinggear shock strut. The desired response from the actuator in thiscondition is to minimize unequal load distribution on the tires andprovide a resistive force that dampens rotations of the bogie beam. Thepressure of the compressible fluid within the inner chamber and thepressure of the hydraulic fluid within the outer chambers are controlledto provide such motive force acting on the pistons to affect aninsignificantly unbalanced load distribution on the tires. Anotherdesired response from the actuator in this condition is to affectdamping of the bogie beam rotations. To dampen bogie beam rotation, thefluid in the inner chamber is forced thru a fluid flow restrictingdevice as the second piston is extended. Additionally, the inner chamberis partially filled with compressible gas and partially filled withhydraulic liquid.

The actuator provided by the different illustrative embodiments, such asactuator 1001 in FIG. 10 and actuator 1102 in FIGS. 11-17, provides adampening effect on the bogie beam during touchdown of the aircraft.Touchdown can cause rapid rotation of the bogie beam and therefore veryrapid linear motion of the actuator pistons. A routine touchdown willaffect extension of the first and third pistons that will forcehydraulic fluid through a restrictive passage and resist motion of thebogie beam. A non-routine, or alternate, touchdown will affectcompression of the first and second pistons that will compress the gaswithin the first fluid chamber. A mechanical spring placed between thefirst and second pistons provides control of accelerations duringcontact between the pistons.

The telescopic hydraulic strut assembly described by the differentillustrative embodiments provides different types of functionality wheninstalled on a conventional multi-axle landing gear and during typicaloperation of an aircraft. In particular, the telescopic hydraulic strutassembly described by the different illustrative embodiments providesthe ability to position the bogie beam in two positions while theaircraft is in flight. The first position is for stowage of the landinggear within a wheel well, and the other position is a positionappropriate for landing the aircraft.

Further, the telescopic hydraulic strut assembly described by thedifferent illustrative embodiments provides the ability to inhibitextension of the actuator, known as lock-up, and hold the forward axleof the bogie beam at a constant distance from the aircraft reference,thereby causing the bogie beam to pivot about the forwardly locatedattachment of the telescopic hydraulic strut as the landing gear shockstrut extends during takeoff. This effectively lengthens the mainlanding gear during takeoff.

Additionally, the telescopic hydraulic strut assembly described by thedifferent illustrative embodiments provides the ability to deactivatethe semi-levered landing gear functions when desired to ensure equaltire loading, maximum braking capability, and optimum dampingperformance. Still further, the telescopic hydraulic strut assemblydescribed by the different illustrative embodiments provides the abilityto dampen rotations of the bogie beam upon touchdown and taxiing.

The description of the different illustrative embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the illustrative embodiments inthe form disclosed. Many modifications and variations will be apparentto those of ordinary skill in the art.

Further, different illustrative embodiments may provide differentfeatures as compared to other illustrative embodiments. The illustrativeembodiment or embodiments selected are chosen and described in order tobest explain the principles of the illustrative embodiments, thepractical application, and to enable others of ordinary skill in the artto understand the disclosure for various illustrative embodiments withvarious modifications as are suited to the particular use contemplated.

What is claimed is:
 1. A hydraulic strut assembly comprising: a housingcomprising an outer cylindrical structure and an inner cylindricalstructure; a first piston positioned between the outer cylindricalstructure and the inner cylindrical structure, wherein an outer chamberconfigured to receive a first fluid is formed between the outercylindrical structure, the inner cylindrical structure, and the firstpiston; a second piston nested within the first piston, wherein theinner cylindrical structure, the first piston, and the second pistonform an inner chamber in which a volume of the inner chamber changeswhen at least one of the first piston and the second piston move and inwhich the inner chamber is configured to hold a second fluid comprisinga gas; and a third piston positioned between the outer cylindricalstructure and the first piston, wherein the first piston, the secondpiston and the third piston are configured to move in a directionparallel to an axis through the housing.
 2. The hydraulic strut assemblyof claim 1, wherein at least one of the first piston and the secondpiston are configured to move such that the gas in the inner chambercompresses as the volume of the inner chamber is reduced.
 3. Thehydraulic strut assembly of claim 1 further comprising: an elongatemember associated with the second piston, wherein the elongate member isconfigured to connect a first sub-chamber of the inner chamber to asecond sub-chamber of the inner chamber in which the second sub-chamberof the inner chamber is formed by a cavity within the second piston andwherein the elongate member is configured to draw any hydraulic liquidthat is part of the second fluid from the second sub-chamber into thefirst sub-chamber when at least one of the second piston and the firstpiston extends.
 4. The hydraulic strut assembly of claim 1, wherein thethird piston is configured to divide the outer chamber into a firstsub-chamber and a second sub-chamber and wherein movement of the thirdpiston changes a volume of the first sub-chamber and a volume of thesecond sub-chamber.
 5. The hydraulic strut assembly of claim 4 furthercomprising: a manifold, wherein the first fluid is configured to flowinto the second sub-chamber of the outer chamber from the manifold andwherein the first fluid is configured to return to the manifold from thefirst sub-chamber of the outer chamber.
 6. The hydraulic strut assemblyof claim 5, wherein the manifold comprises: a plurality of channelsconfigured to allow the first fluid to flow through the manifold; and aplurality of valves configured to control flow of the first fluid fromthe manifold into the second sub-chamber of the outer chamber and fromthe first sub-chamber of the outer chamber into the manifold.
 7. Thehydraulic strut assembly of claim 6, wherein the plurality of valvescomprises: a multi-mode pressure-reducing valve; a first solenoidshut-off valve; a second solenoid shut-off valve; and a multi-modepressure-relief valve.
 8. The hydraulic strut assembly of claim 1further comprising: a spring system associated with at least one of thefirst piston and the second piston, wherein the spring system isconfigured to compress in response to a load applied to the springsystem.
 9. The hydraulic strut assembly of claim 1, wherein the innerchamber is divided into a first sub-chamber and a second sub-chamber andfurther comprising: a seal system associated with at least one of thefirst piston and the second piston, wherein the seal system isconfigured to divide the first sub-chamber of the inner chamber into afirst portion and a second portion and wherein the seal system providesa seal between the first portion and the second portion of the firstsub-chamber when the second piston retracts.
 10. The hydraulic strutassembly of claim 9, wherein the second fluid comprises the gas and ahydraulic liquid and wherein a volume of the first portion of the firstsub-chamber increases and a volume of the second portion of the secondsub-chamber decreases in response to the second piston extending suchthat the hydraulic liquid is forced past the seal system to provideresistance to movement of the second piston.
 11. The hydraulic strutassembly of claim 10, wherein the hydraulic liquid is configured tolubricate the seal system.
 12. The hydraulic strut assembly of claim 1,wherein the first fluid comprises a hydraulic liquid and the secondfluid comprises the gas and the hydraulic liquid in which the gas iscompressible.
 13. The hydraulic strut assembly of claim 1, wherein thehousing, the first piston, the second piston, and the third piston forman actuator in the hydraulic strut assembly and wherein the actuator isconfigured to have a position selected from one of a compressedposition, a retracted position, and a fully extended position.
 14. Thehydraulic strut assembly of claim 13, further comprising: a manifold,wherein the actuator and the manifold are part of a landing gearassembly in an aircraft.
 15. An actuator for use in a hydraulic strutassembly, the actuator comprising: a housing comprising an outercylindrical structure and an inner cylindrical structure; a first pistonpositioned between the outer cylindrical structure and the innercylindrical structure, wherein an outer chamber configured to receive afirst fluid is formed between the outer cylindrical structure, the innercylindrical structure, and the first piston in which the first fluidcomprises a hydraulic liquid; a second piston nested within the firstpiston, wherein the inner cylindrical structure, the first piston, andthe second piston form an inner chamber in which a volume of the innerchamber changes when at least one of the first piston and the secondpiston move and in which the inner chamber is configured to hold asecond fluid comprising the hydraulic liquid and a gas; and a thirdpiston positioned between the outer cylindrical structure and the firstpiston, wherein the first piston, the second piston and the third pistonare configured to move in a direction parallel to an axis through thehousing.
 16. The actuator of claim 15, wherein the at least one of thefirst piston and the second piston are configured to move such that thegas in the inner chamber compresses as the volume of the inner chamberis reduced.
 17. The actuator of claim 15 further comprising: an elongatemember associated with the second piston, wherein the elongate member isconfigured to connect a first sub-chamber of the inner chamber to asecond sub-chamber of the inner chamber in which the second sub-chamberof the inner chamber is formed by a cavity within the second piston. 18.The actuator of claim 15, wherein the third piston is configured todivide the outer chamber into a first sub-chamber and a secondsub-chamber and wherein movement of the third piston changes a volume ofthe first sub-chamber and a volume of the second sub-chamber.
 19. Amethod for operating an aircraft to perform an alternate landing, themethod comprising: operating the aircraft to perform the alternatelanding, wherein an actuator in a landing gear assembly for the aircraftcomprises: a housing comprising an outer cylindrical structure and aninner cylindrical structure; a first piston positioned between the outercylindrical structure and the inner cylindrical structure, wherein anouter chamber configured to receive a first fluid is formed between theouter cylindrical structure, the inner cylindrical structure, and thefirst piston; a second piston nested within the first piston, whereinthe inner cylindrical structure, the first piston, and the second pistonform an inner chamber in which a volume of the inner chamber changeswhen at least one of the first piston and the second piston move and inwhich the inner chamber is configured to hold a second fluid comprisinga gas; and a third piston positioned between the outer cylindricalstructure and the first piston, wherein the first piston, the secondpiston and the third piston are configured to move in a directionparallel to an axis through the housing; and retracting the secondpiston and the first piston in response to a load being applied to thesecond piston when the landing gear assembly contacts a ground on whichthe aircraft is landing, wherein the gas in the inner chamber compresseswhen the second piston retracts.
 20. The method of claim 19, wherein thestep of retracting the second piston and the first piston in response tothe load being applied to the second piston when the landing gearassembly contacts the ground on which the aircraft is landing furthercomprises: retracting the second piston and the first piston in responseto the load being applied to the second piston when the landing gearassembly contacts the ground on which the aircraft is landing such thatthe actuator changes from a fully extended position to a compressedposition, wherein the first piston and the second piston compress thegas that is in the inner chamber.